Explanation:

- Option 1: C + I

- C represents Consumption by households.

- I represents Investment by businesses.

- In two-sector models, equilibrium is when C + I = total income.

- Option 2: C + I + G

- G represents Government spending.

- In three-sector (closed economy), equilibrium adds government spending: C + I + G = total income.

- Option 3: C + I + G + (X-M)

- X represents Exports, M represents Imports.

- In four-sector (open economy) model, equilibrium is when aggregate demand equals aggregate output: C + I + G + (X-M).

- It accounts for foreign sector with net exports (X-M).

- Option 4: None of these

- Suggests that none of the provided options represent the equilibrium condition.

Option 3: C + I + G + (X-M) is indeed correct for the four-sector model, representing the most comprehensive view of an economy that includes households, businesses, government, and international trade.