We don’t know that the police chief “knowingly tolerated” widespread corruption. So (A) would strengthen the argument.

We don’t know that “all” of the chief’s subordinates are corrupt. And we’re trying to conclude that the chief should be held accountable. We’re not trying to prove that the chief must have known about the corruption.

(C) would allow us to conclude that a supervisor should NOT be held accountable, as long as we know they couldn’t reasonably be expected to know about the corruption. But it doesn’t help us prove that someone SHOULD be held accountable. (If you think it does, you’re confusing sufficient and necessary conditions. This answer does NOT tell us that if the supervisor could reasonably be expected to know about corruption, they should be held accountable.)

We’re talking about a chief who has been head of the department for a long time. In any case, (D) allows us to reach a conclusion that someone can’t be expected to take corrective action. But we’re trying to prove that the chief should be held accountable.

(E) gets us from the premise to the conclusion. The chief has been head of the department (so, in a position of authority) for a long time. According to (E), then, the chief has no excuse that can absolve him from responsibility for widespread corruption among subordinates (the people supervised in the department). If he has no excuse that can absolve him from responsibility, that means he should be held responsible.