Answer:
An underlying idea
Step-by-step explanation:
All theme is about is a writer expressing their ideas in their books/poems. Some examples in themes are love, war, good v.s evil, redemption, coming of age, revenge, and courage in someone.
These themes are always found in personal narrative stories because the author writing it is putting their own Point of View in a character they created or you see it in everyday comic books you read.
Therefore it is an underlying idea, so C is your answer
Hope this helped :)