Which economist developed an economic theory that became influential during the global depression as well as the rest of twentieth century?

John Maynard Keynes is the correct answer.

Explanation:

John Maynard Keynes was British Economist and the ideas he propounded are known as Keynesian economics. It  affected the modern political and economic theory and affected government tax and economic policies. He proposed that governments can stop the negative effects of economic recessions by tax and banking measures. He is considered father of Modern Economics. He wrote the book How to Pay for the War during the second world war.

the answer is states.