c. sunk costs are backward-looking and should be ignored when making decisions.
Explanation:
Opportunity costs are the potential benefits you give up by choosing one option over another. They are forward-looking because they represent what you could have gained if you had made a different decision.
Sunk costs are expenses that have already been incurred and cannot be recovered. They are backward-looking because they represent past choices and cannot be changed.
When making decisions, you should focus on the future costs and benefits. Sunk costs are irrelevant to the current situation and should not be a factor in your decision-making process. You should consider the opportunity costs, however, as they represent the potential consequences of your choices.