Final answer:
The total time required for one unit to be completely processed at the workstation with both machines is 6 minutes, as the average provided already includes the time spent on both machines.
Step-by-step explanation:
If the average time for each unit to be processed by both machines is 6 minutes, then the total time required for one unit to be completely processed at the workstation, considering the operation of both machines, is also 6 minutes. This is because the 6-minute average already takes into account the time it takes for both machines to process the unit. Hence, the correct answer is a. 6 minutes.