The correct answer is: A. irrelevant
Sunk costs are costs that have already been incurred and cannot be recovered. They are irrelevant to future decisions because they cannot be changed. For example, if you have already bought a ticket to a concert, the cost of the ticket is a sunk cost. It does not matter whether you go to the concert or not, you will not get your money back. Therefore, the cost of the ticket should not be considered when making a decision about whether to go to the concert.
Option B is incorrect because sunk costs are irrelevant to future decisions. Option C is incorrect because sunk costs are not useful for decision-making. Option D is incorrect because sunk costs are relevant to past decisions, but they are not relevant to future decisions.