@LizD, this is a Strengthen with Necessary premise, so in essence, you're looking for a premise that has to be true for you to reach the logical conclusion in the question, and in this case, it takes the form of ruling out an alternative explanation.
To reach the conclusion that law enforcement got lazy (was considerably more lax) you must rule out that the actual number of violations dropped alongside the number of citations. Without that assumption, which is (A), it is possible that the number of violations dropped that steeply and that law enforcement agents cited the same percentage of cases as before. It's even possible that law enforcement got stricter and cited more but that there were fewer to cite. In both cases, you could not conclude that law enforcement was considerably more lax. However, if you assume that the decrease was not caused by a decreased number of violations, then you can reach the conclusion as stated in the question.
