The correct answer and the reason why Mark Twain used detailed descriptions of Aunt Polly's actions in "The Adventures of Tom Sawyer" is A. To help the reader to form mental pictures.
These detailed descriptions employed by Twain help the reader to imagine how Aunt Polly does look like and what she is doing. By doing this, Twain gives us a well-developed and round character with many facets, a distinctive dialectical voice and a stern but compassionate heart. All of these traits are easily understood by the audience through the detailed descriptions used by Twain.