I don't know which novel you're asking about. What book is it?
D is not your answer. Someone who is bitter and exhausted has learned something that made them bitter and exhausted. They have changed.
C is not your answer. We have a hook here. We're drawn in to find out why she is different. That's not a flat character.
A is not your answer. These twins have a conflict. It is a natural way to develop a story.
B as charming as he sounds, his charm and wit lead us nowhere. He is a foil for the main character. Our pleasure in him is only momentary. B is the answer.
That's incorrect usage.
You'd say "I haven't seen him"
Answer:
Her
Explanation:
Her is a possessive determiner, not a possessive pronoun.