Step 1: Break the word into parts.
The word "protagonist" comes from the Greek "protos" meaning first and "agonistes" meaning actor or competitor. So a protagonist is the lead character, the one the story is built around.
Step 2: Think about what an antonym needs to do.
An antonym must flip the core idea. Here the core idea is "the person the story favors and follows". The opposite idea is "the person who fights against that lead character".
Step 3: Match the prefix.
The prefix "ant-" or "anti-" means against. Adding it to "agonist" (a competitor) gives "antagonist", a competitor who stands against someone. This fits exactly what we need.
Step 4: Rule out the sound-alike options.
Agnostic, arsonist and anarchist all sound close to antagonist because they share letters and a similar ending, but their meanings point elsewhere. Agnostic is about doubt over religious belief. Arsonist is about setting fires. Anarchist is about rejecting government rule. None of them describe someone opposing a story's hero.
Step 5: Confirm the pick.
Only antagonist carries the built in sense of "against the main character", which is the true opposite of protagonist.
\[ \boxed{\text{antagonist}} \]
