Answer:
Option B ("[D]ejected that he was capable, with his Norman cunning, of doing what they had accused him of...") would be the best choice to support the theme of "the folly of pretense" in "A Piece of String." This passage suggests that the character is pretending to be something that he is not, and that he is troubled by the fact that he is able to deceive others with his pretense. This ties into the theme of "the folly of pretense," as it suggests that it is foolish to try to appear as something you are not, and that this behavior can lead to negative consequences.