I am assuming that the correct answer is Choice A. This is a classic inference question where the argument shows 2 contradicting positions. The argument talks about different regulations for teenage drinking in US and Europe, and states that the despite the difference in regulations the in incidence of teenage drinking is similar in both continents.
For the purposes of discussion, I will focus only on choices A and B.

Inference questions are like "must be true" questions, therefore we should look for an answer choice that must be true based on the information in the passage. Lets examine the answer choices.

Choice A: Setting the minimum age for drinking alcohol at 21 cannot be the only factor that affects the incidence of teenage drinking.

Let's use the negation principle. If the minimum age were the only factor that affects the incidence of teenage drinking then we would have definitely see a difference in incidence rates between US and Europe since US has minimum age limit while Europe does not. However, the argument clearly states that in-spite of the difference in regulations, the incidence rates are no different. Hence, there must be other factors that have an impact on the incidence of teenage drinking (such as available of alcohol, cost of alcohol. etc.) Hence, choice A is correct.

Choice B: Setting the minimum age for drinking alcohol at 21 does not play a role in causing teenagers to begin drinking.

This choice takes an extreme position. Note that the argument does not provide us with enough information to infer that the minimum age does not play a role. While you may be tempted to assume the same due to similar incidence rates, there may be other factors that may have neutralized the impact of regulation in US