Final answer:
Direct material costs refer to the costs specifically associated with the materials used to manufacture a product. The cost of steel used to manufacture motorcycle frames and the cost of lubricants used on the production line would be classified as direct material costs.
Step-by-step explanation:
Direct material costs refer to the costs specifically associated with the materials used to manufacture a product. In the case of a motorcycle manufacturer, the cost of steel used to manufacture motorcycle frames would be classified as a direct material cost (option A). This is because the steel is an essential component of the motorcycle frame and can be directly traced to the final product. On the other hand, the salary of the factory supervisor overseeing production (option B) and the depreciation on manufacturing equipment (option C) would be classified as indirect costs or overhead costs. These costs are not directly associated with the materials used in production, but rather with the overall management and operation of the manufacturing process. Lastly, the cost of lubricants used on the production line (option D) can also be classified as a direct material cost, as the lubricants are directly used in the production process of the motorcycles.