This question's pretty tough so I hope you didn't spend too much time on it.
The stimulus tells us that "the law" is as follows. It's important for (C) to note that in context, we are to presume that this is the only law that pertains to contributions to mayoral campaigns in Weston.
What's "the law"?
If (1) $100+ and (2) currently nonresident and (3) never was a resident then must be registered.
For Brimley's campaign, we know that (4) he complied with this law and (5) accepted contributions from residents and former residents and no one else.
What must be true? (4) tells us that his campaign did not run afoul of the law (in other words, no contradiction). We overlook this fact because (4) is a conclusion and we're well trained to be skeptical of conclusions. But, this is a MBT question. The question stem explicitly tells us to presume that EVERYTHING in the stimulus is true. We must accept that in fact Brimley's campaign was run legally.
(5) tells us that Brimley's campaign failed the sufficient conditions of the law. Remember logic games lessons? Sufficient failed, rule irrelevant. In context, that means Brimley's campaign did not have to register any of its contributions. That's exactly what (C) says.
(A) is the attractive, trap answer choice. We're thinking, well, (A) must be true right?
If nonresidents contributed in excess of $100, then it would have to be registered.
First, that's false. This is true: if nonresidents who were never residents contributed in excess of $100, then it would have to be registered. See the difference?
Second, even if that's not false, we don't actually know if Brimely's campaign registered any contributions. We know that (C) they did not NEED to register. But maybe they registered for fun anyway.