The opportunity cost of earning an advanced college degree is that:

Opportunity cost is the loss of potential advantages or gain caused due to you choosing one option from a number of alternate options. The answer to the question is D. You will earn less or no money for the period you are in college. For every choice you make, there is potential benefit as well as harms by choosing that option. As far as opportunity cost regarding choosing a college is concerned, different factors need to be considered. For example:
a. Availability of Jobs: Lets just assume we have jobs available in tons b. Pay scale for such jobs which do not require advanced degrees: lets assume each such available job pays you USD 40k/year in minimum
A college degree typically requires you to stick to college for 4 years. So in these 4 years, you will be sacrificing around USD 160k in total. This is the opportunity cost of going to college. However, this assumption can fail if jobs are not available and takes time to find as well your growth prospect in terms of career growth can be hindered by not having a college degree.

The answer is (D) You will earn less money during the years that you are in college

Basically, opportunity cost can be referred to the loss of potential gain from one choosing one thing over the other or from a number of several options. Assume you choose not to go to college and pick up a job after high school. You get a job that pays $30k per year. 

After 4 years, you’ll have earned $120k. Assume you decide to go to school and not work while in college. The college fee per year is $30k plus expenses. After 4 years, you will be out $120k. The opportunity cost in this case will be $240k since you chose to go to college instead of working. Thus, you will earn less or no money during the years that you are in college.