Explanation:

- Opportunity cost refers to the value of the next best alternative that is foregone when a decision is made to pursue a certain action.

- Option 1: Money cost

- This is the actual expenditure incurred in purchasing goods or services.

- It involves straightforward spending of money.

- Option 2: Real cost

- This includes both tangible and intangible resources used in production.

- Considered in terms of labor, time, and effort instead of monetary cost.

- Option 3: Explicit cost

- These are direct, out-of-pocket expenses that you can see in transactions.

- For example, wages, materials, and rent.

- Option 4: Implicit cost

- These are non-monetary opportunity costs related to using resources owned by the firm.

- Includes potential earnings from alternative uses of the resources.

- Correct Answer: Option 3 - Explicit cost