Why does she expect an answer? Does God typically talk to her and tell her what to do? Probably not. 
I think that denies her from making the choice herself. None of these choices are clearly wrong or right; she has sound and holy reasons for each. She needs to decide on her own. Thus I would suggest that her prayer is instead answered in a different way than she expects. She falls asleep, and wakes in another place, on a path approaching an obstacle or guard, where she can make her way into Faerie via the forked path of choices. God will protect her there as he protects all with True Faith, but it sounds like she needs more time to consider her way. This would certainly give her that.
All the (true) love stories in the setting (courtly love) have the same basic development: they end BAD. They are dramas. Lancelot and Genevieve and Tristan and Isolde spring to mind. CONSUMMATED love outside marriage is something you will eventually pay for, and something you need to avoid at all costs, even if it is soul-splitting to do so.
So I would go for 2 or 3, 2 being the social favorite. That or try to arrange a change of station for the true loved one. help of the covenant, anyone? 
For magi it is relatively easy to meddle in the affairs of mundanes NOT CAUSING damage to their sodales with some choice mentem magics to ensure he raises in station in the local magnate's court. Or to carve a nice fief for the guy out of wild land and ensure he is appointed to the place as the rightful conqueror/tamer of nature. That would indebt the dude heavily in favor of the covenant, so it might be seen as an investment by the more practical-minded magi out there that do not give a damn about love and such worthless things (worthless because they cannot be affected by Hermetic magic, that is)
So you swallow your disgrace and live with it or truy to change the situation. Or you have tyour sister marry the other guy and you downgrade yourself, get the black sheep flaw and marry below your status. It will be a scandal for sure, but much worse has been seen. The Tudors in the war of the roses, for example.
Cheers,
Xavi
Or, since it's True Love, God changes the parameters - aka the SG cheats
Because she has faith?
Some could argue that faith doesn't need an answer...
Perhaps she marries as family decides (dutiful daughter and part of what women were supposed to do in this period) and then something happens to the husband she doesn't love and she becomes widow. Now she has his lands and can marry the knight as second husband.
If not, there were many a tragedy where true love did not get to marry. If she is pious, then option 3 is not an option. She is not in love with God that she wants to become a bride of christ. She is in love with a man. The love is pure but acting on it (1 or cockholding the husband) is sinful. Marry the noble and indulge his sinful lusts to begat children and love truly and do what you can to benefit your love knowing it can never be consumated (a sin).
This is probably the 'best' choice
An excellent summation.
That's agnostism.
Receiving guidance from the divine is a miraculous effect-- I don't think it's something the player can expect to just happen. If God speaks to her, or sends an angel to lead her or whatever, that's a special thing. Granted, the storyguide can decide to just make a miracle happen, possibly because of the importance of the occasion, but I think it should be clear that this is what has happened.
What the divine would want for her seems like it would be pretty uncertain in this situation. None of these options are inherently sinful, so I think it should come down to what is most important to her. Obviously she doesn't want to become a nun, so that probably isn't even an option. So how can she choose between her love for her family and her people, and her true love? The divine can't make that choice for her, I would say. Thus, I would expect the miracle she receives to be something else, something that would help her make her choice on her own.
Certainly is not.
However I will qualify what I said.
Faith does not require an explicit answer. Faith manages. Those of strong faith tend to find answers and meaning in other things unrelated to their question or problem.
Of course not, players know about game mechanics.
But a character with faith believes that god (or a saint or an angel or whatever) will miraculously intervene.
Exactly. Faith expects to receive an answer. If no answer appears forthcoming, a character with faith looks harder for the answer.
Looking strictly at the mechanics, Milady is in trouble. She needs her True Love around, or else she will sink into melancholy.
That said, at the Storyguide's discretion, any acts she takes to be united with her True Love can get a bonus of up to +3, and any acts he takes can benefit likewise.
Beyond that, Milady has True Faith. If anyone can expect some Divine instruction or Saintly visitation, she can. And if she wants to make a speech tomorrow about how God has fore-ordained her hand to another and she just can't consent to the wedding, if I were SG I'd allow a full +9 bonus stacking Confidence, True Faith, and True Love on top of each other. Her father may not like it, but if Milady can manage a good enough total everyone who hears it might acknowledge her Divine remit. At the very least her father might need to delay the wedding while he hunts down a priest willing to officiate.
Running off to a Nunnery and delaying full vows is a more traditional tactic, but anyone who is sincerely pious is a possible candidate for getting a message from On High that Milady is in need of a helping hand.
-Albert
She does have a fourth option that is not listed.
It is a variant on c.
She can go on Pilgrimage to some holy site or holy man. Going to speak to the pope, pray at some very remote shrine, speak to godly man performing miracles (Maybe Francis of Assisi if she is right time frame and he already been doing miracles for God.)
This Giving God time to work, time to think over options and time to consult someone truly holy. It is not a permanent committing herself to god but buys her time for the other lord to perhaps find another bride, for her to think it over, for her to persuade her family to her view (if father or mother comes with her), for something to happen to the lord she would have been married to. The Lord works in Mysterious ways and pilgrimages were very much the thing in the times.
This also gives option of appeal to higher authority than family to command family to do the holy thing for true love and remove conflict between family duty and sacred love.
If the pope says that marriage to the knight for the true love for example is the pious and holy thing, I suspect her family will most definately will allow her to marry the knight.
Excellent option!
It depends which are the player characters in this and what the SG thinks would give them the greatest pleasure. I'm assuming below that the Lady or the Beloved or both are PCs. If they are background characters then you get slightly different answers.
If they want to play out a story of True Love that only bears fruit after years of patience or not until they meet again in the next world then that's one thing. She marries the man she does not love for duty and the True Lover must pine from afar... Unless hubby is very understanding....
If they want a story of True Love given triumphant justication by God then have angels (subtly or blatantly) arrange for the marriage to fail at the last moment. Perhaps the True Love is coming to beg her to run away with him and on the way rescues a strange knight from enemies and that just turns out to be the betrothed husband....
Myself, I don't like to give players solutions so much as chances to show how full of awesomeness their characters are. Let them have a clearly outlined scene in which they have a way to triumphant if they do the right thing amazingly enough or come up with something even more wonderful than you can.
How about a scene in which the Lady meets the True Love one last time and tells him that she must marry, it is her Duty and God himself has revealed that she must consent... She sends him away... And an angel has guided the Betrothed's steps to the garden to overhear two True Lovers express their Love for each other...
If they do it awesomely enough then he might be moved to give up his claim. If either of them acts with less than real grace he bursts from the undergrowth and...
Let it be decided by the quality of the role-playing and of the dice rolls....
EDITED TO ADD: And in all events make the Betrothed a decent sort of chap within his limits. Probably never given a thought to what the woman he would marry would feel about it in his life but willing to be gracious. It would help if he had a good reason to want the marriage to go ahead as well so that he doesn't give her up lightly (if he does).
And on another note could it turn out that the young landless and impoverished knight is actually his long-lost son? There's something you could twang heartstrings with!
I seem to be in a minority position in thinking #3
The relevant passage is from 1 Corinthians, Chapter 7, verse 8 and 9
And 1 Corinthians, Chapter 7, verse 34 and 35
and Matthew 10:34-37
So, the absolute best choice is to become a nun, and devote herself to God. God's claim on her is greater than the claim of her father.
Also, in an error of trial by combat, the knight potentially could challenge the fiance for the right of marriage. God is on his side if the love is true.
Just another idea and possibility
Although the church officially frowns on trials by combat (and other ordeals) in 1220. At this time they are considered an archaic barbarism. At least that is the position of the church; the divine might have a different opinion.