Your opportunity cost of taking this course is

multiple choice $2$

the cost that you are paying to take advantage of this opportunity such as the portion of your tuition that pays for this, the time spent studying, and the time spent in lecture.

the cost of attending a course you could not take because you are taking this one.

the opportunity that you gain from taking this class such as the knowledge, the skills, the pleasure of your professor's company, and the opportunity to compete with your fellow students.

the benefit of the next$-$best alternative; that benefit becomes a cost because you forgo one activity by taking this course.