Subject/Verb agreement is one of the most commonly tested concepts in GMAT SC.

While "audience" is composed of a lot of people, it's actually SINGULAR. That's what's called a "collective noun" -- a group of individuals that functions as a single entity. Other examples of collective nouns would be: group, family, team, etc.

Because "audience" is singular, we can't use the plural verb "are". We need "is" instead. Eliminate (A) and (B). [Also, "in soccer matches" is a prepositional phrase, so it can't contain the subject....so "audience" is the subject that will get the verb "is".]

(E) makes it seem like we're talking about just 1 audience in 1 soccer match.

The only difference between (C ) and (D) is "frequently" vs. "sometimes". Both are grammatically correct, but "frequently" matches the *degree* that we had with "oftentimes" in the original sentence.

So the correct answer is (C).