(A) still doesn't tell us anything more about the distribution of meteorite strikes, so doesn't guarantee the author's conclusion that geophysical processes are the key factor in where we find meteorite craters.

In other words, a region that's currently stable might have been unstable millions of years ago. But this actually undermines the argument by diminishing the distinction between geologically stable and unstable regions. (B) doesn't guarantee the author's explanation, but in fact makes it weaker by limiting the explanatory power of geophysical processes.

In other words, if (B) is true, then we wouldn't really expect to find more craters in stable regions because those regions would also have been affected by geophysical processes in the past. Over time, all regions would be stable and unstable in turn, so the stability of a region becomes less able to explain why we find impact craters there.

Without knowing about the distribution of these meteorites, the rate at which they strike doesn't affect the argument. And without affecting the argument, (C) can't guarantee the conclusion.

This is the answer eliminating a competing explanation that we were looking for. (D) tells us that meteorite distribution isn't the reason for impact crater distribution; if meteorites are distributed evenly, we need geophysical activity to explain why stable regions have more craters.

You might notice that (D) doesn't eliminate all possible competing explanations—for example, the consideration raised in (E), that geologists focus more on some regions than others. Although we would normally expect a higher level of certainty in a Sufficient Assumption answer, (D) is still the best we have. It's still correct because it gets us very close to a guaranteed conclusion.

Like (B), (E) undermines the argument; in this case, (E) actually proposes a competing explanation. If we've studied stable regions more, maybe they don't actually have more craters—we've just found more of them. This makes the author's conclusion less likely.