The correct answer is C. Raw materials cost . A cost incurred as part of the production process is a product cost , and raw materials are one of the most direct examples. In manufacturing, raw materials are inputs physically used to create finished goods, so they are clearly part of production. Manufacturing cost guidance consistently identifies direct materials, direct labor, and manufacturing overhead as production-related costs.

Option A, sunk cost , refers to a past cost that has already been incurred and cannot be changed, so it is a decision-making concept rather than a specific production input. Option B, opportunity cost , represents the benefit forgone by choosing one alternative over another, not an actual recorded production cost. Option D, period cost , refers to costs such as selling and administrative expenses that are expensed in the period incurred and are not part of the manufacturing process. Because raw materials are directly consumed in producing goods, they are the clearest example of a cost incurred as part of production. Therefore, the correct answer is Raw materials cost , making Option C correct.