The correct answer is: A. Long-run average cost (LAC) curves.
A plant curve is a long-run average cost curve. It shows the lowest average cost of producing a given level of output when the firm has had time to adjust all of its inputs, including the size of its plant.
A short-run average cost (SAC) curve shows the lowest average cost of producing a given level of output when the firm cannot adjust all of its inputs. In the short run, the firm is limited by the size of its plant and other fixed costs. As a result, the SAC curve is typically U-shaped.
An average variable cost (AVC) curve shows the average cost of producing a given level of output when the firm varies all of its inputs except for labor. The AVC curve is typically U-shaped because the marginal cost of labor is initially decreasing and then increasing.
An average total cost (ATC) curve shows the average cost of producing a given level of output when the firm varies all of its inputs. The ATC curve is typically U-shaped because the average variable cost curve is U-shaped and the average fixed cost curve is always positive.