The correct answer is: C. I, III, IV
The minimum Long Run Average Cost (LAC) can be determined on a LAC curve for a normal production function and a planning curve. A linear production function does not have a minimum LAC. An envelope curve is the long-run average cost curve that is tangent to all short-run average cost curves.
A normal production function is one that exhibits increasing marginal returns at low levels of output, followed by diminishing marginal returns at higher levels of output. The LAC curve for a normal production function is U-shaped, with the minimum LAC occurring at the point where the LAC curve intersects the average variable cost (AVC) curve.
A planning curve is a graph that shows the minimum cost of producing each level of output. The planning curve is derived from the LAC curve by considering the costs of all inputs, including fixed costs. The minimum LAC on the planning curve occurs at the point where the planning curve intersects the AVC curve.
A linear production function is one in which the output is proportional to the inputs. The LAC curve for a linear production function is a straight line, with the minimum LAC occurring at the point where the LAC curve intersects the AVC curve. However, a linear production function does not exhibit diminishing marginal returns, which is unrealistic.
An envelope curve is the long-run average cost curve that is tangent to all short-run average cost curves. The envelope curve is the lowest possible LAC curve that can be achieved, given the technology and input prices. The minimum LAC on the envelope curve occurs at the point where the envelope curve intersects the AVC curve.