Final answer:
The cost of quality control supervisors' training is classified as a prevention cost, which is a proactive measure to ensure product quality and prevent defects.
Step-by-step explanation:
The cost of training quality control supervisors who are responsible for overseeing the inspection of units would be classified as a prevention cost. Prevention costs are part of the four categories of quality costs, which also include appraisal costs, internal failure costs, and external failure costs.
Prevention costs are incurred to prevent defects from occurring and include things such as training employees, implementing quality systems, and doing design reviews to prevent flaws in products or processes. Training quality control supervisors is a proactive measure to ensure that products meet quality standards before they reach the customer, therefore helping to prevent defects and errors.