The answer is D. Supervision cost.
Direct labor expenses are the costs of labor that are directly associated with the production of a good or service. They include the wages and salaries of production workers, as well as the costs of benefits such as health insurance and retirement contributions.
Inspection cost, testing cost, and assembly cost are all direct labor expenses. They are all costs that are incurred in the production of a good or service and that are directly attributable to the labor that is used to produce the good or service.
Supervision cost is not a direct labor expense. It is a cost of management that is incurred in the production of a good or service, but it is not directly attributable to the labor that is used to produce the good or service.
For example, the wages of a production worker who assembles a product are a direct labor expense. The wages of a supervisor who oversees the production of the product are not a direct labor expense.