Opportunity cost is

A) what we give up to get something else.
B) marginal cost divided by total cost.
C) the same as real cost.
D) all of the above.

Opportunity cost refers to the value of the next best alternative that is foregone when making a decision. It represents what is given up to obtain something else, making choice A correct. Choices B and C describe different economic concepts and are not definitions of opportunity cost.