When using management by exception, the purchasing manager should be questioned for the direct materials cost variance. The direct materials cost variance measures the difference between the actual cost of direct materials used and the standard cost of direct materials that should have been used. It is an important variance because it can help identify inefficiencies in the purchasing process, such as paying too much for materials or buying materials that are not needed. The other variances listed (direct materials efficiency, variable overhead efficiency, and direct labor efficiency) are also important, but they are not directly related to the purchasing manager's responsibilities.
The answer is C.