D. The characters' behaviors, motivations, and growth.
Usually the theme of a story is not stated directly, the author shows the theme by how the characters grow and change. If you're reading about a character who starts off young, immature, or naive and by the end of the book he is mature, grownup, and more knowledgeable about the world the theme might be "Coming of Age". If a character goes through the book and people around him are dying--based on how the main character changes or reacts the theme might be "The Power of Death".
Pay close attention to the main characters, how they change and grow, and look for themes wrapped within those changes.
