The correct answer is: D. planning curves.
Long-run cost curves are called planning curves because they show the costs of production for all possible levels of output when all inputs are variable. In other words, they show the minimum cost of producing any given level of output when the firm has had enough time to adjust all of its inputs.
Operating curves, on the other hand, show the costs of production for a given level of output when some inputs are fixed. This means that they do not show the minimum cost of production, but rather the cost of production given the firm’s current level of investment in fixed capital.
Fixed curves and variable curves are not commonly used terms in economics. However, if they were used, they would likely refer to the same thing as operating curves and planning curves, respectively.
In conclusion, long-run cost curves are called planning curves because they show the costs of production for all possible levels of output when all inputs are variable.