The correct answer is A. Sunk cost.
A sunk cost is a cost that has already been incurred and cannot be recovered. It is not relevant to future decisions, and should not be considered when making a decision.
B. Marginal cost is the additional cost incurred when one more unit of a good or service is produced.
C. Incremental cost is the difference between the total cost of two alternatives.
D. Replacement cost is the cost of replacing an asset.
Sunk costs are often difficult to ignore, because they can be emotionally charged. However, it is important to remember that sunk costs are irrelevant to future decisions. When making a decision, you should only consider the costs and benefits that will occur in the future.