Answer: The answer is "D. average total costs are falling".
Explanation: The average cost curve is derived from the total cost curve, since it is the total cost curve divided by the number of units.
As for the relationship between the average cost curve and the marginal cost curve, when the marginal cost is less than the average cost, the average cost curve is falling. On the contrary, when the marginal cost curve is larger than the average cost curve, the latter is growing. When the Marginal Cost is equal to the Average Cost, it is at its minimum point and therefore the CMe curve will be flat at that point.