The correct answer is C. Sunk cost.
A sunk cost is a cost that has already been incurred and cannot be recovered. It is a cost that has been paid and cannot be changed, regardless of future decisions. Sunk costs are often irrelevant to future decisions, because they cannot be changed. For example, if you have already paid for a concert ticket, you cannot get your money back if you decide you do not want to go to the concert. In this case, the cost of the ticket is a sunk cost and should not be considered when making a decision about whether or not to go to the concert.
Marginal cost is the cost of producing one additional unit of a good or service. It is the change in total cost that results from producing one more unit. Out-of-pocket cost is the actual amount of money that is spent on something. It is the cost that is incurred immediately, without any delay.
None of these options is a sunk cost.