The correct answer is D.
Opportunity cost is the cost of an alternative that must be forgone in order to pursue a certain action. In economics, it is the value of the next-best alternative forgone when making a choice.
For example, if you choose to go to college, the opportunity cost is the income you could have earned if you had worked instead. Or, if you choose to buy a new car, the opportunity cost is the money you could have saved or spent on other things.
Opportunity cost is important because it helps us to make rational decisions. When we consider the opportunity cost of our choices, we are more likely to make choices that are in our best interests.
A. Cost related to an optimum level of production is not the definition of opportunity cost. The optimum level of production is the level of production at which the marginal cost is equal to the marginal revenue.
B. Variable cost is not the definition of opportunity cost. Variable cost is the cost of production that varies with the level of output.
C. Short-run cost is not the definition of opportunity cost. Short-run cost is the cost of production that is incurred in the short run, when some inputs are fixed.