A story's main theme might be difficult to figure out because:

AAuthors rarely want the reader to figure it outincorrect answer
BIt has little to do with plot, characters, and other story elementsincorrect answer
CIt is rarely stated explicitlyincorrect answer
DOnly complicated, adult stories have themes

D) Only complicated, adult stories have themes

This is incorrect. Themes can be present in all types of stories, not just complicated adult ones. Themes are universal ideas that can be found in various genres and styles of writing.