Answer:
The correct answer is letter "A": The opportunity cost of teaching an introductory course is higher for graduate students than for full professors.
Explanation:
Opportunity cost represents the return of the choice taken compared to the return of the option left behind. For a university professor, the opportunity cost of giving an undergraduate class is lower than the opportunity cost that reflects teaching the undergraduate class to graduate students. This is because graduate students are starting to obtain experience but professors have it already.