Official Explanation Magoosh:
If a person is wearing a bicycle helmet, then, all things being equal, that’s person injury would be less severe than had they not been wearing a helmet. The thing is we can’t assume all things are equal between one who wears a helmet and one who does not wear a helmet.
Therefore, to properly evaluate the argument, we have to be able to show that those wearing helmets were engaging in similar activities as those not wearing helmets. For instance, if those who wore helmets tended to go down steep trails at high speeds and those who did not wear helmets rode around the neighborhood, the argument is not valid. This leads us to (D).
(A) does not talk about the severity of accidents. We want to know if there is a fundamental difference in the severity of accidents between the helmet clad and the non-helmet clad.
(B) superficially seems as though it would work. We don’t know if those admitted to the emergency room are wearing old helmets. If so, then it would be important to know (B), since helmets that were old and no longer offered protection would compromise the argument.
(C) is wrong because the conclusion is not concerned with the number of injuries. Rather, it's concerned with the severity of injuries and so this answer choice is out of scope.
(E) does not relate to the issue: are the two groups—the helmet wearing and the non-helmet wearing—equivalent?
FAQ: Is this a strengthening/weakening question? How do we approach questions like this?A: This kind of question: "which would be most useful to evaluate the argument?" involves understanding the assumptions for the argument. The correct answer choice will tell us whether a key assumption for the argument is true or not. Basically, the question is asking us what information we need to know in order to decide whether the conclusion of the argument is valid.
For this question, the argument is that bicycle helmets reduce injury, therefore cyclists who were wear helmets will have less severe injuries.
One assumption of this argument is that cyclists with helmet do not engage in more risky/dangerous behavior. So, to evaluate the argument we would need to know answer choice (D) "whether the bicycling activities of cyclists using a helmet differs from the activities of those not wearing a helmet."
FAQ: Why is choice B wrong?A: The problem with (B) is that we aren't given any information regarding the age of the helmets in question. Maybe all these helmets are new -- we just don't know. So (B) doesn't help us as much as (D).