The Antonym of the word ”Protagonist” is_______.

The correct option is Antagonist.

To understand why this is the correct antonym, let us break down the definitions and etymology of the words:
1. Protagonist: Derived from the Greek roots proto- (meaning "first" or "primary") and agonistes (meaning "actor" or "combatant"). In literature, drama, or real-world scenarios, the protagonist is the leading character, hero, or champion of a particular cause or story.
2. Antagonist: Combined from the Greek prefix anti- (meaning "against" or "opposite") and agonistes. An antagonist is a person who actively opposes, struggles against, or competes with another; in a story, this is the adversary or rival of the protagonist.

Since the protagonist is the central figure and the antagonist is the character who acts in direct opposition to them, these two words are direct opposites (antonyms).

Let us also examine the other choices to see why they do not fit:
- Agnostic: A person who believes that nothing is known or can be known of the existence or nature of God.
- Anarchist: A person who rebels against established authority or ruling powers, advocating for a society based on voluntary cooperation without government.
- Arsonist: A criminal who intentionally starts fires to destroy property.