Explanation:

- Option 1: Stock variable

- A stock variable represents a quantity measured at a particular point in time.

- Examples include wealth, capital, and inventory.

- The supply of money is a stock variable.

- Option 2: Flow variable

- A flow variable represents a quantity measured over a period of time.

- Examples include income, expenditure, and production.

- Option 3: Real variable

- A real variable is adjusted for inflation and reflects the true purchasing power.

- Examples include real GDP and real wage.

- Option 4: None of the above

- This option is invalid in this context, as the money supply fits within the discussed categories.