The correct answer to the question is option 3, John Maynard Keynes.

Deficit financing refers to the practice of a government spending more money than it receives in revenue, resulting in a budget deficit. This concept was first proposed and advocated by John Maynard Keynes, a prominent economist during the early 20th century.

Keynes believed that during times of economic downturns or recessions, the government should increase its spending to stimulate economic activity and employment. This would involve running budget deficits, which would be financed through borrowing or printing additional money. Keynes argued that deficit financing could help boost aggregate demand, which in turn would lead to higher output and employment levels.

The other options listed are not correct. Adam Smith, a Scottish economist, laid the foundations for modern economic theory but did not specifically propose deficit financing. Alfred Marshall was an influential economist known for his work on microeconomics, but he did not suggest deficit financing either. Milton Friedman, a prominent economist of the 20th century, advocated for free-market policies and monetarism, but he did not propose deficit financing as a policy measure.

Therefore, the correct answer is option 3, John Maynard Keynes.