What does opportunity cost mean?
Opportunity cost is a term that indicates the benefits, or opportunities, a person or business must give up in order to pursue a certain action or decision. Opportunity cost is often the result of some kind of scarcity (ie lack of time, money, resources etc.)
In this situation the action we are pursuing is to earn an advanced college degree, thus, the opportunity cost of earning an advanced college degree is any action and/or decision we can not take because we are busy earning this advanced college degree.
The answer to this problem is:
A. You will earn less money during the years that you are in college.
In the situation proposed in the question, you are scarce on time, because you choose to spend it earning your degree and studying. You can not use the time you spend going to college to also work and earn money, thus you are not earning as much money as you could were you not in college (not in college = more time you could spend working and earning money)
Here is a further breakdown of why the other responses are incorrect:
B. You will earn more income during your career.
This is a delayed benefit (not a cost) you'd receive after working hard on a degree that could land you a stable career
C. Some fields require a professional degree before you
This is one motivation possibly pushing you to get your degree
D. There may be a low supply of jobs in your professional field.
This is a risk you may encounter