What is theme?

A. the dramatic struggle or clash between two or more forces in a story

B. the recurring object, concept, or idea in a story

C. the lesson a reader infers from reading a story

D. the central message or insight revealed through a story

The correct answer is D. the central message or insight revealed through a story

A theme is usually something simple like "good always wins against evil" and this can be explored through various stories and poems, but in the end it's actually the central idea regardless of the medium.