"A character sketch is a description made for the purpose of guiding the writer on such assignments as introducing a particular character to the reader in such a way that both the writer and readers can have a quick overview of the character's attributes; including but not limited to physical appearance, idiosyncratic (individual) tendencies in different circumstances, state of mind as shaped by former experiences and how these will affect their future actions."
Option B: The character sketch may or may not be formed of the main characters. It can also be about a character who plays a minor but pivotal role, depending on the story itself. Hence there is no compulsion, and option B is correct.
Options A, C and D: The character sketch is not limited to only the main characters, protagonist or antagonist. It can be on a person who's not the main character but plays an important role at some point.
Hence options A, C and D are wrong.