An overwhelming proportion of the most productive employees at SaleCo's regional offices work not eight hours a day, five days a week, as do other SaleCo employees, but rather ten hours a day, four days a week, with Friday off. Noting this phenomenon, SaleCo's president plans to increase overall productivity by keeping the offices closed on Fridays and having all employees work the same schedule-ten hours a day, four days a week.
Which of the following, if true, provides the most reason to doubt that the president's plan, if implemented, will achieve its stated purpose?

For weakening questions, we need to attack the conclusion.The correct answer choice will not simply contradict the conclusion. Instead, the correct answer will undermine the conclusion, by showing that the conclusion fails to account for some element or possibility.

A. Typically, a SaleCo employee's least productive hours in the workplace are the early afternoon hours.
We are not concerned knowing which are the least or most productive hours. We are concerned with the relation between working 4 days a week and increased productivity

B. None of the employees who work four days a week had volunteered to work that schedule, but all were assigned to it by their supervisors.
This is a strengthener.
If employees work better when work is assigned by others, then this new plan imposed by an other person ( here president) can also make everyone productive

C. Working ten hours a day has allowed the most productive employees to work two hours alone each day in their respective offices relatively undisturbed by fellow employees.

This answer choice finds a gap.This answer points to show that the conclusion in the stimulus has failed to account for the possibility that the most productive employees may be productive because they got to work two hours alone each day in their respective offices relatively undisturbed by fellow employees.Now, if everyone works 10 hours a day, no one will be able to work alone any time , and hence there is a high chance that no one will be productive any more.

D. Employees at SaleCo are compensated not on the basis of how many hours a week they work but on the basis of how productive they are during the hours they are at work.
We are not discussing compensation here. We are talking about how everyone can be productive.

E. Those SaleCo employees who have a four-day workweek do not take any of their office work to do at home on Fridays.
This may be true. But this choice does not discuss how the four days a week work schedule for all will impact everyone's productivity

Answer: C