Concept:Marginal cost is the extra cost incurred when one additional unit of output is produced.Explanation:It is calculated as the change in total cost divided by the change in output.In symbol form, MC=ΔQΔTC.When output rises by one unit, marginal cost equals the increase in total cost.Fixed costs remain unchanged with output, so option A is incorrect.Options B and C do not define marginal cost correctly.Answer:D. increase in total cost as output is increased by one unit