Official Explanation

This argument features issues with two extremely common logical fallacies: 1) correlation vs. causation and 2) data pools that aren't necessarily comparable. If you see these elements in the gap in logic, you can anticipate the right answer.

First, notice that in the 20 years between arrest statistics that the official cites, many things could have occurred other than the laws she cites. What if, for example, alcohol tariffs made the price so exorbitant that everyone just quit drinking? Or the city built a system of canals and everyone just kayaks around town now? There could well be other causes for the statistic - the laws might be correlated with the time period, but did they really cause the outcome?

Second, notice that the use of actual-number data (25,000 arrests vs. 18,000 arrests) doesn't necessarily tie to the conclusion. Yes the number of arrests down (and the total population is up), but the conclusion is that the legislation was successful in "preventing motorists from driving under the influence." Since "motorists" is a subset of the total population, you'd really want to see a statistic that isn't just total number of arrests, but something more like arrests per 1,000 motorists" - a statistic that accounts for the fact that the number of motorists could be way down (in which case "motorists" - those who still drive - might still be driving under the influence quite frequently, but the overall statistic is down because there are simply much fewer drivers).

Given those errors in the argument, choice (D) is correct - it shows that the number of motorists is down, and supplies an alternate cause for the drop in the number of arrests. People are using Lyft and taking the train, not driving anymore.

Among the other choices:

(A) is irrelevant, as whether the population has grown at a high or low rate compared to other provinces, the fact remains that the population has still increased. (And really what you want to know is the number of drivers/motorists)

(B) seems like it should matter (is this a good use of money?) but remember that the specific conclusion is only about whether the laws worked, not about whether they were a wise use of funding. Always stay within the specific scope of the conclusion!

(C) misses the mark because of its timing - the statistic used in the argument is about arrests, and (C) notes that this intervention of lawyers occurs after the arrests have already taken place. If lawyers were acting before the arrests, that might suggest that the lawyers are causing the reduction in the number even though people are still drinking and driving, but that's not the case here - the lawyers in (C) don't come into the picture early enough to explain away the number of arrests.

And (E) is similar - if the number of establishments serving alcohol were way down that might be part of an alternate explanation for the reduction in arrests, but with the number of restaurants and pubs serving alcohol increasing, that's not the case.