should be (B)
D looks much more appropriate to me. Can you describe your thought process behind B?
1. Impact craters have been found in the greatest density in geologically stable regions. Why?
2. Because of lower rates of destructive geophysical processes in these geologically stable regions.
This conclusion (1) can hold iff the rates of destructive processes are known to be the same in all geological regions, in which case if the rates are high, then impact craters will not be found in substantial amounts (as well as the opposite). When you negate (B), my choice, you get:
Rates of destructive geophysical processes within any given region DO NOT vary markedly throughout geological time. How does this affect the conclusion?
If the rates of destructive geophysical processes do not vary, then you cannot say for sure (as the argument states) that the rates are solely responsible for the occurence of impact craters in certain regions.
Refuting (D) by negation:
Actual meteorite impacts have NOT been scattered fairly evenly over the Earth’s surface in the course of Earth’s geological history.
IMO, that is what the argument already tells you - that there is unequal distribution of impact craters in regions of earth (which are on the surface of earth). So, it's not an assumption.
Hope i'm correct after all these.