Explanation

D, the key (a perverse incentive): the measure's apparent success is what defeats it. Once exams feel secure, instructors move more of the grade onto unsupervised work. That raises the payoff for cheating where most cheating already happens, so the extra proctors are offset or worse. This is the same kind of reasoning as the source key: a safety measure changes behavior and increases exposure to the risk it doesn't address.
A is a detection-bias reversal. It sounds like a sophisticated point about unreliable records. But if exam cheating is undercounted, exams make up a larger share of real dishonesty than the records show, and proctors would matter more. So it weakens the argument.
B shows that proctors are effective, which points toward the proposal working. It is a mild weakener.
C connects exams to out-of-class cheating. But adding proctors changes nothing about exam pressure, so it is neutral.
E is a sampling trap. The records may be biased, but nothing says in which direction, so it neither strengthens nor weakens.