The protagonist of a writing or work is the main character, also considered to be the good guy or hero in most circumstances. This is because even if a villain was a main character, you would be seeing the story through their eyes, a story to which they will most likely believe themselves to be good and righteous in their own way. Remember that an antagonist would be the opposing force to the central character or protagonist.
The answer, therefore, would be B. the central character who is the focus of interest.