Concept: The correct word must describe stepping into a situation to stop or change it.
Explanation: The sentence describes a man who was knocked down after trying to stop a fight.
"Intervene" means to become involved deliberately in a situation to influence or prevent an outcome.
This fits the action of stopping a fight.
"Intrude" means to enter without invitation.
"Interpose" means to place oneself between things.
"Obstruct" means to block progress.
However, only "intervene" clearly conveys stepping in to halt the fight, which is why it is the best choice.
Answer: C. intervene