A theme is the central topic a text treats.Themes can be divided into two categories: a work's thematic concept is what readers "think the work is about" and its thematic statement being "what the work says about the subject".
With that being said, A would be the main idea of the story; B would be a summary or conclusion, and C would be the author's purpose. So, your answer would be D. Hope that helps and makes sense to you. (:
