The correct answer is C. Slope of AC.
Marginal cost (MC) is the change in total cost (TC) that arises when one additional unit of output is produced. It is calculated by taking the derivative of TC with respect to output.
Average cost (AC) is total cost divided by output. It is calculated by taking the total cost function and dividing it by the output level.
The slope of the AC curve is the change in AC that arises when one additional unit of output is produced. It is calculated by taking the derivative of AC with respect to output.
The slope of the TC curve is the change in TC that arises when one additional unit of output is produced. It is calculated by taking the derivative of TC with respect to output.
The slope of the TFC curve is always zero, because total fixed cost does not change when output changes.
Therefore, the correct answer is C. Slope of AC.