(A) gets us from the premise to the conclusion. “Only if” introduces a necessary condition. So (A), restated, means that if a supervisor does NOT know that all the work performed by an employee can be performed equally well by another employee, then the supervisor should NOT request the employee to be replaced. We know from the premise that Ted performs some “unique” work that is “possibly irreplaceable.” This establishes that Tatiana doesn’t know with certainty that she can replace Ted’s work with another employee. (A), then, would justify the decision not to request that Ted be replaced.

Leads to wrong conclusion. We’re trying to support the decision not to request Ted be replaced. (B) helps us reach a conclusion about how employers should compensate employees.

Leads to wrong conclusion. (C) would help us conclude that someone doesn’t have the authority to decide whether Ted should be replaced. But we’re trying to support the conclusion that there shouldn’t be a request that he be replaced. Whether someone has the authority to replace Ted is a separate issue.

Leads to wrong conclusion. (D) would help us conclude that workers should feel responsible for certain work. But we’re trying to prove that Ted shouldn’t be requested to be replaced.

Not strong enough. (E) establishes that when measuring an employee’s contributions, we have to take into account more than just the time that person works. That doesn’t help us prove that Tatiana shouldn’t ask for Ted to be replaced. (Don’t get confused about what we’re trying to prove. We’re not trying to prove that Ted makes meaningful contributions despite his short working hours.)