Explanation

In our initial approach, we can see that we have a question asking for a logical continuation. The most important material seems to start with the phrase "uncertain of reaching either population in its entirety." That morsel, plus the fact Country B is smaller, gives a good reason to administer in Country B; it's otherwise equal, but denser (we might guess we are assuming they have equal populations?), so we can administer more efficiently.

Applying the filter: Choice (A) talks about population density, so it passes the filter. Choice (C) also seems related, because it involves country size, but it's in fact introducing a completely new idea. So it's not a logical continuation, as the blank demands; what goes in the blank doesn't fit as a completely new reason, but will have identifiable logical grounding in what has come earlier in the prompt. Choice (A) is the only relevant candidate in this respect. We confirm the logic. They are basically still talking about how Country B is smaller. Not only is that country smaller, but while Country A's transportation is a little better, which would help administration of the vaccine, it's not better enough to outweigh the fact that Country B is smaller and there is a population density favoring administration. Answer choice (A) is a logical continuation indeed. The correct answer is (A).