The conclusion of this argument is, "This relatively greater abundance of securely identified craters in geologically stable regions must be explained by the lower rates of destructive geophysical processes in those regions." The support for this conclusion is that impact craters have been found in the greatest density in geologically stable regions.
This is a cause and effect argument. The observed effect that the author is trying to explain is the higher number of impact craters in geologically stable regions. The proposed cause? The lower rates of destructive geophysical processes in those regions.
Right away we should notice that this argument is flawed. The author is overlooking the possibility that the reason we have found impact craters more often in geologically stable regions is that meteorites actually hit these regions of the Earth more frequently.
This is a Strengthen with Sufficient Premise question so we are trying to guarantee the conclusion that the reason for this difference in density is in fact due the the lower rates of destructive geophysical processes in those regions.
Answer choice (D) guarantees the conclusion by eliminating our possible alternate explanation, i.e. that meteorites actually hit these regions of the Earth more frequently than others. If it is true that actual meteorite impacts have been scattered fairly evenly over the Earth's surface in the course of Earth's geological history, then it must be true that the difference in the density is due to the lower rates of destructive geophysical processes in those regions.
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