Final answer:
Elizabeth doubts Miss Bingley's defense of Darcy's actions toward Wickham because she has been influenced by Wickham’s own account and observes Miss Bingley's bias due to her affections for Darcy. Option C.
Step-by-step explanation:
Elizabeth does not believe Miss Bingley when she tries to defend Darcy's actions toward Wickham because she has heard Wickham's side of the story first, which was quite convincing and painted Darcy in a negative light.
Wickham's charming and seemingly open nature contrasted with Darcy's pride and reserve could have influenced Elizabeth's readiness to believe Wickham over Miss Bingley.
Moreover, Elizabeth has observed Miss Bingley’s behavior and noted her obvious partiality towards Darcy, which could call into question her objectivity and credibility in defending Darcy's actions.
So option C.