Generally, supervisors look over the work of their assigned employees, meaning how they function and basically piece it together. Since he receives frequent requests, he should train the workers on how to fix the machines if they're not working properly. The reason I didn't pick option "A" was because John receives frequent requests, and supervisors have more work to do than to move around and fix machines all day.
Training the workers would have a better result in the end.
Answer: B. John should make sure he trains the workers better, including how to fix the machine.