Which of the following correctly defines a literary theme?
a. the main topic of the story
b. a short summary of the characters and events of a story
c. a brief explanation the author's purpose for writing the story
d. a universal lesson taught within the story about life or the way people behave

 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.  (: