Man must ...... to stop pollution.

Answer: Option A

Man must act to stop pollution.
In this context, "act" is the most appropriate verb to convey the idea of taking action to address or mitigate pollution. "Act" implies taking deliberate steps or measures to achieve a particular goal, which aligns with the intention of stopping pollution. Therefore, "act" is the correct choice to complete the sentence, accurately conveying the necessity for proactive intervention to combat pollution.