Correct Option: D

Classical economics is widely regarded as the first modern school of economic thought. Its major developers include Adam Smith, Jean-Baptiste Say, David Ricardo, Thomas Malthus and John Stuart Mill. John Maynard Keynes was a British economist whose ideas have profoundly affected the theory and practice of modern macroeconomics, and formed the economic policies of governments. He built on and greatly refined earlier work on the causes of business cycles, and is widely considered to be one of the founders of modern macroeconomics and the most influential economist of the 20th century. His ideas are the basis for the school of thought known as Keynesian economics, as well as its various offshoots.