Which statement best explains this excerpt from act v, scene i, of twelfth night? sebastian: [to olivia] so comes it, lady, you have been mistook: but nature to her bias drew in that. you would have been contracted to a maid; nor are you therein, by my life, deceived; you are betroth'd both to a maid and man. sebastian explains that the case of mistaken identity has worked in olivia’s favor because she has married sebastian, not viola. sebastian says that olivia has mistakenly married a woman instead of a man, which goes against the "bias" of nature. sebastian is mocking olivia because she married someone without really knowing that person's identity. sebastian is congratulating olivia for marrying a great man and hiring a maid to with her household. sebastian is unwilling to get married and tells olivia that it would be a mistake to expect him to do so. nextreset

This scene comes at the end of the play, as all has been revealed. In this scene, 

Sebastian says: "So comes it, lady, you have been mistook." By this he means that Olivia has been mistaken, or wrong. He continues, "But nature to her bias drew in that." By this he means, nature has fixed your error and made it right.
 
He explains, "You would have been contracted to a maid," meaning that Olivia would have married a maiden, or a young woman. However, he continues by saying, "Nor are you therein, by my life, deceived; You are betroth'd both to a maid and man." He means that Olivia is not entirely deceived, or not entirely wrong. He says that Olivia married both a young woman AND a young man. He does not mean this literally, of course, since that isn't quite what happened. Instead, he means that the love Olivia had for Sebastian's sister has been transferred into a love for him. And, since he is still a virgin, he is a "maiden" as well as a man.

Now, let's look at the statements:

Sebastian explains that the case of mistaken identity has worked in Olivia’s favor because she has married Sebastian, not Viola.
-- This statement best summarizes Sebastian's words to Olivia. 

Sebastian says that Olivia has mistakenly married a woman instead of a man, which goes against the "bias" of nature.
-- Olivia has not mistakenly married a woman; therefore, this statement would not be correct.

Sebastian is mocking Olivia because she married someone without really knowing that person's identity.
-- Sebastian is not mocking Olivia at all; instead, he is trying to explain how Olivia's mistake worked in their favor after all.

Sebastian is congratulating Olivia for marrying a great man and hiring a maid to help with her household.
-- Olivia has not married a maid to help with her household; this statement as a result is not correct.

Sebastian is unwilling to get married and tells Olivia that it would be a mistake to expect him to do so. 
-- This is not at all what he is saying, as he is in love with Olivia; this statement would not be correct.