I would say the correct answer is B. She believes her affair with Tom confers status upon her.
Myrtle is having an affair with a married man, Tom. She is also married, to a poor owner of a car-shop. Therefore, when she started cheating on her husband with Tom, who is wealthy and respectable, she started having a better opinion of herself, thinking that she has become a lady now that she is in a relationship with a 'gentleman.'