When they started the fighting, the man who tried to … was knocked down.

Step 1: Analyze the question The sentence is: "When they started the fighting, the man who tried to ____ was knocked down." We need a verb meaning to step into a fight to stop it or make peace.

Step 2: Explain the concepts

• Intrude: to enter a place or situation uninvited, where you are not wanted. It carries the idea of disturbing privacy and does not imply stopping a fight.
• Interpose: to place something or someone between two parties, or to interrupt with a remark. It usually needs an object ("interpose a barrier"), and it can be used intransitively, but it is less natural here.
• Intervene: to step in to a dispute or conflict in order to stop it or change the outcome. It is the standard word for stepping into a fight or quarrel.
• Obstruct: to block or hinder something, such as a road or progress. It requires an object and does not fit.

Step 3: Apply to the sentence "The man who tried to intervene" means the man who tried to step in and stop the fight. It is natural that he got knocked down by the fighters. The phrase "tried to intervene" is a very common collocation.

Step 4: Answer The correct option is C. intervene.

Tip: Remember the prefix inter- means "between." Intervene is the everyday word for stepping between people in a conflict. If the context is a fight, argument, or dispute, think "intervene." Use intrude when someone is unwelcome, and obstruct when something is being blocked.