Weston has a law about which campaign contributions must be registered with the city council. The law applies to a specific category:

Required to register: contributions over $100 from nonresidents of Weston who are not former residents.

Brimley's campaign accepted contributions only from residents and former residents. Since none of those contributions fall into the registration category, the campaign complied with the law.

This question turns on a move that's rare on the LSAT. Normally, when a stimulus says "all X must Y," that's a one-way conditional: if X, then Y. It doesn't tell us anything about non-X. By default, you should read this law that way: contributions in the described category must be registered, with the door left open for other contributions to also need registration for reasons the stimulus doesn't mention.

But the stimulus says the law of Weston regarding contributions to mayoral campaigns is as follows. Not "a" law. Not "one" law. The law. That signals we're being told the entire rule on the subject, not just one provision among others. If contributions outside the described category also needed to be registered, we'd expect to be told. So we can read the law as covering both directions: contributions need to be registered if and only if they're over $100 and come from nonresidents who aren't former residents.

This two-way reading (also known as a biconditional) is unusual on the LSAT. The default is to treat "all X must Y" as one-way, and that default is right in almost every case. Here, the "the law... is as follows" framing is what allows us to treat the rule as a biconditional. If the stimulus had said something like "one law of Weston requires...," we wouldn't do that.

Under the biconditional reading, contributions from residents and former residents never need to be registered. Since Brimley's contributions came only from those two groups, we can prove that no contributions to Brimley's campaign needed to be registered.