The correct answer is $\boxed{\text{A. }\$40,000}$.
Prime cost is the sum of direct material cost and direct manufacturing labor cost. Direct material cost is the cost of the materials that are used directly in the production of a product. Direct manufacturing labor cost is the cost of the labor that is directly involved in the production of a product.
In this case, we are given that prime cost is $50,000 and direct manufacturing labor cost is $10,000. Therefore, direct material cost must be $50,000 – $10,000 = $\boxed{\text{A. }\$40,000}$.
Option B is incorrect because it is the total prime cost, not the direct material cost. Option C is incorrect because it is the sum of direct material cost and direct manufacturing labor cost, which is equal to the prime cost. Option D is incorrect because it is half of the direct material cost.