Final answer:
A relevant cost is a cost that makes a difference in decision-making. It is a cost that can be changed and affects the outcome of a decision.
Step-by-step explanation:
A relevant cost is a cost that makes a difference in decision-making. It is a cost that can be changed and affects the outcome of a decision. In contrast, a sunk cost is a cost that has already been incurred and cannot be changed or recovered.
For example, let's say a company is considering whether to continue producing a product or discontinue it. The relevant costs in this case would include the variable costs like raw materials and labor that would change depending on the decision. The sunk costs, on the other hand, would be the fixed costs like machinery and equipment that have already been incurred and cannot be changed.
Therefore, option C is the correct answer as a cost is considered relevant when it makes a difference in decision-making.