The correct answer is C. Round curve.
A long-run average cost (LRAC) curve is a graph that shows the lowest average cost at which a firm can produce any given level of output in the long run. It is the envelope of all the short-run average cost curves.
A planning curve is a graph that shows the lowest average cost at which a firm can produce any given level of output in the long run, given the firm’s expectations about future prices and costs.
An enveloping curve is a graph that shows the lowest possible cost at which a firm can produce any given level of output, given the firm’s technology and the prices of inputs.
A round curve is not a name for an LAC curve.