The answer is C because when she says this part in the play, Romeo is trying to swear his love to her, but she is saying that although she is thrilled that he is in love with her, she cannot find joy in his promise because she feels that a promise that large should not be made "too rash, too unadvised, too sudden". The part about lightning is her saying that the promise is comparable to lightning; it flashes and then leaves before you can even say what it was.
