The answer to your question would be that when an author focuses on characterization in a piece of fiction, it means that he or she provides details about the different people in the story. That is, your answer would be C.
Characterization is a literary device that is used to highlight and explain the characters in a story. First, the author introduces the character, then he or she talks about his behaviour and, finally, the author provides information about his/her thought processes. What is more, characterization also has to do with the characters expressing their ideas and opinions and with how the rest of the characters react to the character's personality. It also involves taking into account the conversations they have.