Correct Option: A

In economics, marginal cost is the change in the total cost that arises when the quantity produced is incremented by one unit. That is, it is the cost of producing one more unit of a good. Marginal cost is independent of the fixed cost and depends on the changes in the variable factors. Since fixed costs do not change with output, there are no marginal fixed costs when output is increased in the short run. It is only the variable costs that vary with output in the short run. Therefore, the marginal costs are in fact due to the changes in variable costs, and whatever the amount of fixed cost, the marginal cost in unaffected by it.