Explanation:

- Option 1: A.C. Pigou

- A British economist known for his work in welfare economics.

- Associated with the Pigovian tax to correct externalities.

- Option 2: Malthus

- Refers to Thomas Malthus, famous for his theories on population growth.

- Known for "An Essay on the Principle of Population".

- Option 3: J.M. Keynes

- Correct Answer

- Authored "The General Theory of Employment, Interest, and Money".

- Founder of Keynesian economics, shaping modern macroeconomic policies.

- Option 4: Marshall

- Alfred Marshall was a pioneer in microeconomics.

- Known for "Principles of Economics" and concepts like elasticity and consumer surplus.