I selected D for this originally. No idea how I fell for it, as it's a fairly common trap in LR. It doesn't matter what so and so believe - is it true or not?
Anyway, upon review, it's clear to me why B is correct. After all, if the nightclub patrons would suspect that anybody who refrains from drinking is a police officer, then of course the exception to the rule of 'No police officers are allowed to drink' is justified. If there is no such exception to the rule, then the entire operation of undercover investigating is undermined. The officers aren't exactly undercover if everybody's thinking to themselves that the guys aren't drinking are officers. What's one aim when going undercover? Fitting in.
But I still also find A appealing. Maybe others do as well. So I thought this rundown of A could be helpful to me and others.
There's no doubt that B is a better answer, and it provides a more concrete reason for justifying the exception. But if only the very experienced police officers are permitted to work undercover, then it would seem we'd have more reason to justify the exception. That said, I see why A is wrong. It's inadequate, because we have to bring in too much information to assume that very experienced officers can hold down their liquor better and that it doesn't inhibit their work to any significant degree. We can just as reasonably go the other way with the assumption, in assuming that the very experienced officers are jaded with their jobs, and so are more likely to abuse alcohol! Or maybe these officers think they have nothing to lose due to enhanced job security, so they drink recklessly. Who knows? Given this uncertainty, A isn't the best justifier.
Even still, could this not be a right answer to a question like this? Yeah, it's not that good, but often times on the LSAT, you'll see a question where all the answers suck and it's the one that sucks least that wins out. It seems to be a very commonsense assumption that more experienced people merit our trust relative to less experienced people. So theoretically, could this not justify the exception if B didn't provide a better answer?