The person, idea, force, or set of circumstances that opposes the central character of a story is the

motif

protagonist

central conflict

antagonist

(D) Protagonist

Wrong - A: motif is a symbol or technique
Wrong - B: Antagonist presents opposition to the main character. ("Anti" Protagonist = Antagonist)
Wrong - C: Central conflict is the conflict/relationship between the protagonist and antagonist. 

The person, idea, force, or set of circumstances that opposes the central character of a story is the antagonist. This is the character which has the main conflict with the protagonist or the main character of the story. For example, the protagonist of the story Cinderella are the step mother and the step sisters