Answer:
The sentence that uses omission correctly is:
C. One serious omission in the team list was the name of the coach.
Explanation:
Omission is a noun, which allows us to eliminate option B since the sentence is using it as a verb. Omission means failing to include something or someone. When you omit, you leave out, you exclude. Having that meaning in mind, we can easily eliminate options A and D, since the context in those two sentences does not allow for the use of omission.
Letter C is the best option. It uses omission as the noun it is, and the context and the meaning are a match. According to the sentence, leaving the name of the coach out of the team list was a serious mistake.