1. The problem asks: What makes a cost relevant in decision-making? 2. A relevant cost is one that will affect future decisions because it differs between alternatives. 3. Important rules: - Past costs (sunk costs) are not relevant because they cannot be changed. - Fixed and common costs are usually not relevant if they do not differ between alternatives. - Only costs that are future and differ between options should be considered relevant. 4. Therefore, the correct definition is: A cost is relevant if it is future and different under alternatives. 5. This means the cost will impact the decision because it varies depending on the choice made. Final answer: Future and different under alternatives