<span>The question is asking for the best revision of the sentence' Mustafa ran 100 meter and 200 meter races and the one he had trained for the hardest was the 400 meter competition. The best revision is this: Mustafa ran the 100, 200 and the 400 meter races competition; the one he had prepared hardest for was the 400 meter race.</span>
The best revision for this sentence is Mustafa ran the 100-meter and 200-meter races (but the one he had trained for the hardest was the 400-meter competition).
We have the use of parenthesis to present the additional information about the 400 meters race, we use parenthesis in the same way we can use commas or dashes to separate incidental or less important information.
