The correct answer is D. Marginal cost curve.
Average fixed cost (AFC) is a per-unit cost that declines continuously as the quantity produced increases. This is because the fixed costs are spread out over a larger number of units.
Average variable cost (AVC) is a per-unit cost that first declines and then increases as the quantity produced increases. This is because the variable costs increase at a decreasing rate initially, but then increase at an increasing rate as the quantity produced increases.
Average total cost (ATC) is a per-unit cost that is equal to the sum of average fixed cost and average variable cost. It is U-shaped because average fixed cost declines continuously while average variable cost first declines and then increases.
Marginal cost (MC) is the change in total cost that results from producing one additional unit of output. It is always positive and increasing. This is because the marginal cost of producing an additional unit of output includes the cost of the additional inputs that are required to produce that unit, as well as the opportunity cost of using those inputs for other purposes.
In conclusion, the marginal cost curve is not a U-shaped curve. It is always positive and increasing.