sleeping or going out for dinner, whichever she would have preferred the most.
Explanation:
Opportunity cost is the value of what you forgo when making a decision. In Amy's case, she has 1 hour without appointment, but she should use this time studying, sleeping or leaving for dinner. If she decided to spend this time studying, she made a decision by giving up rest and dinner. From this we can conclude that if she decides to study for an hour, the opportunity cost of the time spent studying is sleeping or going out to dinner, whichever she prefers.
