The student returned the book she checked out to the school library before completing her project to avoid a paying a fine. What was her opportunity cost?

Remember that in economics an opportunity cost is the price you have to pay for choosing one thing instead of another. In other words, it is a benefit, or value of something that is given up to adquire something else.

As resources are allways limited, it is important to take very well thought decisions, choosing one implies not choosing other. In this case the opportunity cost of the student is the extra information she could have obtained from the book to improve her project.