Answer: The $45,000 salary that you would be able to earn after having completed your graduate program.
Explanation:
The opportunity cost is an economic term used to describe the cost of making a choice over another.
In the case of the question if I were earning $30,000 at my job, and had to quit the job to further my studies my opportunity cost would be the salary and benefits of the job am leaving and also the expense I would undertake in the graduate program.