Final answer:
The student's opportunity cost of returning the book was the benefit of completing her project using that book, which might have led to a better grade or a deeper understanding of the material.
Step-by-step explanation:
The student's opportunity cost of returning the book to avoid a fine is the value of the best other use of the book they could have had. In this case, that would likely be the benefits received from completing the project using the book as a resource. If this was the most valuable use of the book to the student, then the opportunity cost would be the potential higher grade or deeper understanding of the subject matter that the student might have achieved by keeping the book longer.
This concept is often explored when considering the costs of attending college, where students must weigh the out-of-pocket costs against the lost earnings for the time spent in class and studying. The opportunity cost includes these potential earnings and represents a significant factor in the overall cost of a college education.