Theme in a piece of literature is

A. a message about life and human behavior

B. a lesson stated at the end of a story

C. an opinion of the reader

D. the introduction of a story

The theme is A, a message about life and human behavior. Sometimes, the theme of the lesson isn't directly stated, but is implied throughout the story, so it can't be B. Opinions of the reader are opinions of the piece of literature, and have nothing to do with the literature directly. And the introduction is the beginning paragraph of the story, which the theme is displayed throughout the story, not just one part.