This is an evidence question, only with an atypical format for the answer choices. Each one is simply a statement; it doesn't begin with "Whether." The argument hinges on the difference between students at rural and suburban schools and those at urban schools. The result mentioned in the question is that the difference between those two groups of students, in terms of driving under the influence, is explained by the distances between housing and sources of liquor. We're looking for a choice that would either strengthen or weaken that claim. Consider each choice in turn:

(A) If we knew that "all" students didn't purchase alcohol from these sources, this might cast some doubt on the claim, but "many" leaves an awful lot to be desired. It doesn't directly affect the argument.
(B) This is a different comparison--students vs. non-students. We need a choice that focuses on the comparison drawn in the passage.
(C) Enrollment doesn't matter, since the argument is phrased in terms of likelihood that a certain student would drive under the influence, not the absolute number of students.
(D) This is correct. The conclusion was drawn based on police records. If this choice is true, students driving under the influence near rural and suburban universities are more likely to be caught doing so (and thus appear in police records) than their counterparts at urban universities.
(E) The density of bars and liquor stores doesn't matter; it doesn't address the distance traveled, or amount driven under the influence, of the students in these areas.