This is a weaken question. The plan's goal is to increase revenues; the method of doing so is to increase mid-week DVD rentals. The plan appears to rest of two premises: Extended return dates will bring in more customers during the week, and those customers will rent again on Friday when they return their mid-week rentals. To attack the argument, we only need to attack one of those two claims. Consider each choice in turn:

(A) This isn't relevant. If the plan is implemented, more clerks could be hired.
(B) This is correct. The argument rests on the claim that mid-week renters will rent again on Friday. This choice suggests that that is unlikely.
(C) This doesn't matter. The argument is concerned with those who do rent during the week.
(D) This doesn't directly weaken the argument, which relies on the inducement to renters on Monday and Tuesday, as well.
(E) This is outside the scope of the argument, which is limited to whether the plan under consideration would be effective, not whether it is the most effective possible plan.