Explanation:

- Closed Economy:

- A closed economy is one that does not engage in international trade.

- Domestic income is equal to national income because there are no imports or exports.

- All income generated is from domestic production and consumption.

- This is the correct answer.

- Open Economy:

- An open economy engages in international trade.

- Domestic income may not equal national income due to imports and exports.

- National income includes net income from abroad.

- Both a and b:

- This option is incorrect because only a closed economy ensures domestic income equals national income.

- Neither a nor b:

- This option is incorrect because a closed economy satisfies the condition.