The correct answer is A. A sunk cost is a past cost that cannot be recovered.
A sunk cost is a cost that has already been incurred and cannot be recovered. It is a cost that is not relevant to future decisions, because it cannot be changed. For example, if you buy a ticket to a concert, the cost of the ticket is a sunk cost if you decide not to go to the concert. You cannot get your money back, so the cost of the ticket is irrelevant to your decision of whether or not to go to the concert.
Option B, an opportunity cost, is the cost of something that you could have done if you had not chosen to do something else. For example, if you choose to go to a concert, the opportunity cost is the cost of the other things that you could have done with that time and money.
Option C, the cost of drilling certain types of well for water, is a type of sunk cost. However, it is not the only type of sunk cost. Any cost that has already been incurred and cannot be recovered is a sunk cost.
Option D, a cost that is highly relevant for decision-making, is not a sunk cost. A sunk cost is a cost that is not relevant to future decisions.