We can characterize the claim that low attendance would have led to the museum closing permanently as the idea that having more than low attendance was necessary for the museum to remain open. But the author doesn't think that having more than low attendance guarantees that the museum will stay open. The author isn't even trying to prove that the museum will stay open. We already know that it stayed open. What the author is trying to prove is whether staying open required media coverage. (A) would have been correct had the author made the following argument: "In order for the museum to stay open, it needs more than low attendance. It has more than low attendance. Therefore, it will stay open."

The trustee claims that media coverage likely helped boost this exhibit's record attendance, but he doesn't assume that no other exhibit has ever gotten as much media coverage.

The trustee thinks that local media coverage contributed to the exhibit’s record-breaking attendance, but he doesn’t assume that most people who read articles about the exhibit also attended it. We don’t even know if the “local media” was in the form of articles at all.

The exhibit might have had enough visitors to stay open even without media coverage. Just because the media coverage likely helped draw record attendance doesn’t mean that the exhibit would have had low attendance— and thus closed— without it.

This is the cookie-cutter flaw of circular reasoning, where the author’s conclusion simply restates a premise. The trustee doesn’t make this mistake; his premises and conclusion are distinct.