The opportunity cost of an action: can be determined by considering both the benefits that flow from as well as the monetary costs incurred as a result of the action. can be determined by adding up the bills incurred as a result of the action. can be objectively determined only by economists. is a subjective valuation that can be determined only by the individual who chooses the action.

Your question asks what an opportunity cost of an action is.

The reason why answer choice "D). is a subjective valuation that can be determined only by the individual who chooses the action" is correct because an opportunity cost of an action is not the same for everyone.

An opportunity cost of an action is subjective, meaning that the action can be determined by someone's opinion, feelings, etc. Everyone thinks differently, therefore making everyone's opportunity cost of action different.

A opportunity cost of an action also is determined by the individual themselves, not anyone else. That's why the action is subjective, due to the fact that the decision on the action is determined by the individual personal opinions and feelings.