Answer:
D). Mustufa ran the 100-meter and 200-meter races (but the one he had trained for the hardest was the 400-meter competition).
Step-by-step explanation:
The last option i.e. option D correctly revises the given sentence. It employs the punctuation mark of parentheses or brackets aptly to contain the information that is unessential to the key point that is being stated in the sentence. In this sentence, the main point is that Mustufa took part in a 100 and 200 meters race, and the training she did for 400 meters is non-essential to it.