@Meredith Great question, these two answer choices can be tricky to distinguish between.
Ultimately, Thomas is arguing that Jeffrey should be able to vote because only members in good standing can vote, and Jeffrey is a member in good standing. Note how the phrase "only" good standing members can vote differs in meaning from that of "all" good standing members can vote. This is the flaw in the argument. We see it mirrored in Althea's response: She points out that just because "only" citizens can vote, this does not mean that "all" citizens (including her 2-month-old niece) can vote. So, Thomas is taking the absence of something being prohibited as though it is authorized (answer choice A). This answer choice is more specific to the prompt than answer choice C, making it the better answer.
Another way to think about it is to focus on the wording in answer choice C. For example, C says "if a statement is not actually denied by someone," but in neither Thomas's nor Althea's examples is "someone" actively affirming or denying statements. Choice C also says "that [a] statement MUST be regarded as true" in any case where someone does not deny it. This is strong language that could be taken to extend the reach of Thomas's reasoning. In other words, we cannot assume that Thomas believes we must always regard undenied statements in this way simply because that principle may apply in this specific case.
I hope this helps. Please let me know if you need any more clarification!
