The correct answer is D, Wages.
The cost of production associated with the labor is Wages.
When employees are hired, they are given salaries in exchange of their time and work they give to the organization. So when workers work in a manufacturing firm, they are given wages. Wages are considered as cost of production. Production process involves employees, and employees are given money to do work, so a cost is associated for their work, this is the production cost, called Wage.