Explanation:

- Option 3: Study of national income

- This is a key aspect of macroeconomics. It looks at the overall economic picture, measuring the income generated by the nation and evaluating its economic performance.

- Correct Answer

- Option 1: Private production is promoted

- This concerns microeconomics, which deals with individual and business decisions, not the broader economic issues like promoting private sector production.

- Option 2: Scarcity and choice are studied

- This is a foundational concept in microeconomics, exploring how individuals and small entities make choices due to limited resources.

- Option 4: None of these

- This would be incorrect, as Option 3 correctly relates to macroeconomics.