The correct answer is: C. Below average cost curve.
A firm’s average cost curve is a graph that shows the average cost of producing a product, as a function of the quantity produced. The marginal cost curve is a graph that shows the additional cost of producing one more unit of a product.
If a firm’s average cost curve is falling, then the marginal cost curve must be below the average cost curve. This is because the marginal cost of producing one more unit of a product is always less than the average cost of producing all of the units that have already been produced.
For example, let’s say that a firm produces 10 units of a product at a total cost of $100. The average cost of producing each unit is therefore $10. If the firm produces 11 units, the total cost will be $110. The marginal cost of producing the 11th unit is therefore $10. The average cost of producing 11 units is $100/11 = $9.09. Since the marginal cost is less than the average cost, the marginal cost curve must be below the average cost curve.
Here is a diagram that illustrates this relationship:
[Diagram of average cost and marginal cost curves]
The average cost curve is the blue line, and the marginal cost curve is the red line. The marginal cost curve is always below the average cost curve.
I hope this explanation is helpful. Let me know if you have any other questions.