Final answer:
Elizabeth's shame and vexation are due to her realization of her own prejudices and mistaken judgments about Darcy, which becomes a pivotal moment in her character development. OptionB.
Step-by-step explanation:
Elizabeth is "overpowered by shame and vexation" when Darcy appears unexpectedly at his home because she realizes she has been gossiping about him and reflecting on her prejudices and misconceptions surrounding his character.
This moment serves as a crucial turning point in Jane Austen's novel, where Elizabeth begins to confront her own mistakes and misjudgments regarding Darcy, thereby undergoing a significant change in her feelings and perceptions.
So option B.