Final answer:
Charlotte believes Mr. Darcy is in love with Elizabeth based on the special interest he shows in her, which is observed in the social interactions within the novel.
Step-by-step explanation:
Charlotte believes Mr. Darcy must be in love with Elizabeth because of A) Mr. Darcy has shown special interest in Elizabeth. This observation is based on the social dynamics and interactions presented in Jane Austen's novel. Throughout the story, Mr. Darcy's behavior towards Elizabeth Bennet changes significantly, and he often finds himself inexplicably drawn to her, displaying a special kind of attention that suggests romantic interest. Unlike other options listed, there is no direct public declaration of love, expensive gifts, or mentions in letters that would lead Charlotte to this conclusion.