Correct : b

A protagonist is the leading character or main figure in a story, debate, or cause — the one who drives events forward and whose goals the narrative follows. The word comes from Greek "protos" (first) and "agonist" (actor or combatant).
Its direct antonym is antagonist — the character or force that opposes the protagonist, creating conflict. Antagonist comes from the same Greek root "agonist" but with the prefix "anti" (against), literally meaning one who fights against the main actor.
The other options are distractors that rhyme or sound similar but have completely different meanings — agnostic refers to one who is uncertain about the existence of God, arsonist is someone who deliberately sets fires, and anarchist is someone who rejects organised authority and hierarchy. None of these carry the meaning of "opponent of the main character."
Correct answer: B — Antagonist ✓