Mustufa ran the 100-meter and 200-meter races (but the one he had trained for the hardest was the 400-meter competition).
This is the only answer that is grammatically correct. Parentheses cannot just be place anywhere in a sentence. They are used to add extra information for clarity or information that is an aside). An aside is something that is not the main part of the discussion. In this sentence, the main point is that Mustufa ran the sprint races. The extra information is that he trained for the mid-distance race.
The answer would be “physics”
