The sentence appears to be the theme of a story. Theme explains the message or meaning behind a story, therefore the answer would be B. "Explains the meaning of a story."
If it was the topic, tone, or mood of a story it would be only one word instead of a sentence, so A is incorrect. This sentence tells us nothing about the setting or time period of the story, so D is incorrect. C could have been a possible answer but a summary would better tell what the story was about.
