The correct answer is: A. Opportunity cost.
Opportunity cost is the cost of an alternative that must be forgone in order to pursue a certain action. In other words, it is the value of the best alternative that is given up when a decision is made.
For example, if you decide to go to college, you are giving up the opportunity to work full-time and earn money. The opportunity cost of going to college is the money you could have earned if you had worked instead.
Opportunity cost is an important concept in economics because it helps us to make rational decisions. When we are faced with a choice, we need to consider the opportunity cost of each option in order to make the best decision for us.
Ghost cost is a term used in accounting to refer to a cost that is not actually incurred, but is recorded for accounting purposes. Ghost costs are often used to allocate costs across different departments or products.
Horizon cost is a term used in project management to refer to the cost of carrying out a project over a longer period of time. Horizon costs can include the cost of financing the project, the cost of maintaining the project, and the cost of decommissioning the project at the end of its life.
Null cost is a term used in economics to refer to a cost that is zero. Null costs are often used to represent the cost of doing nothing.
In conclusion, the correct answer is: A. Opportunity cost. Opportunity cost is the cost of an alternative that must be forgone in order to pursue a certain action. It is an important concept in economics because it helps us to make rational decisions.