Answer:C. the highest valued alternative that must be given up to engage in an activity.
Explanation:
Let us say you had a very high paying job with all the benefits such as lucrative salary, medical aid and other benefits but you have to give this up in order to go and study further to improve your education background.
Opportunity cost occurs when you have two alternatives but you have to choose one whilst giving up the benefits of the other alternative which is not chosen but this is done with an aim that in the future the chosen alternative is likely to give you even more benefits.
So if you give up your work to go and study further it is likely that you will get even better opportunities in the future with even more and better benefit but in a mean while you will be losing the salary that you have been getting in your given up job.