Concept:The long-run average cost curve is an envelope tangent to the short-run average cost curves.Explanation:• Each short-run average cost curve represents one fixed plant size.• The long-run average cost curve is tangent to these short-run curves.• However, it touches a SAC curve at its minimum point for only one plant size, the optimum scale corresponding to the minimum point of the long-run average cost curve.• For other short-run curves, the tangency point is not at their minimum.Answer:C. minimum point of only one of the short-run cost curves