I was pretty quick to eliminate A.) because the first premise was prescriptive (ie. contained "should"), which is not the case for the premise in the conclusion. I assume, then, that prescriptive/descriptive gap is not sufficient to eliminate an answer?

I still can't wrap my head around this question, and some explanation would be appreciated. Here is my analysis.

Premise 1: Principle of LF economics: increase minimum wage, decrease in job supply.

Premise 2: Principle fails in the case of fast food restaurants.

Conclusion: LF economics is not ENTIRELY accurate.

For me, the conclusion follows perfectly from the premises because of the modifier "ENTIRELY." This is because premise 2 is an example in which the principle of LF economics fails. Therefore, it is not entirely accurate. Even if premise 2 said "in A restaurant" instead of fast-food restaurantS, one instance of failure is enough to say that the theory is not ENTIRELY accurate. If a theory is entirely accurate, then would not hold for ALL cases?

Therefore, B looked very unattractive me and it still does. Even if the study were not generalizable to other minimum wage jobs in many industries, it doesn't change the fact that it failed for a given industry and is thus not ENTIRELY accurate.