Mustufa ran the 100-meter and 200-meter races but the one he had trained for the hardest was the 400-meter competition. which is the best revision of this sentence? mustufa ran the 100-meter and 200-meter races (but the one he had trained for the hardest) was the 400-meter competition. mustufa ran the 100-meter and 200-meter races but the one he had trained for the hardest (was the 400-meter) competition. mustufa ran the 100-meter and 200-meter races but (the one he had trained for the hardest was the 400-meter competition). mustufa ran the 100-meter and 200-meter races (but the one he had trained for the hardest was the 400-meter competition).

There are two thoughts in this sentence that we're going to form. First, Mustufa participated in the 100- and 200-meter races. Second, Mustufa practiced the hardest during training for the 400-meter race. These thoughts should be placed together without changing their respective meanings. The parenthesis is incorporated to add an additional clause but is not necessarily needed. Hence, when you see a parenthesis, this statement may not be read. You should note that the sentence must be complete even without reading the words inside the parenthesis. Therefore, the answer is 

Mustufa ran the 100-meter and 200-meter races (but the one he had trained for the hardest was the 400-meter competition).