The option that best completes the gap is "if he had run fast."
The sentence is in the past tense, "Usman would have won the race," which means that the missing phrase should also be in the past tense. "If he had run fast" is the correct form of the third conditional, which describes a hypothetical situation in the past that did not happen. In this case, the sentence suggests that Usman did not win the race, and the missing phrase is describing a condition that would have had to be met in order for him to win.
"Only if he could run fast" is not the correct form of the third conditional, and it suggests a present ability that Usman may or may not have.
"If he can run faster" is the incorrect verb tense for the sentence, as it is in the present tense and not the past tense.
"Although he ran fast" suggests that Usman ran fast but still did not win the race, which is not consistent with the idea that he would have won if a certain condition had been met. This makes this option an inappropriate choice for completing the sentence.