Step 1: Identify the Book.
"The General Theory of Employment, Interest and Money," published in 1936, is attributed to John Maynard Keynes.
Step 2: Summarize the Contribution.
This seminal work established the basis of modern macroeconomics. Keynes departed from classical economics, asserting that aggregate demand is the primary driver of overall employment and output, particularly during economic downturns.
Step 3: Disqualify Incorrect Options.
- (A) Adam Smith → Author of "Wealth of Nations" (1776).
- (B) David Ricardo → Known for "Principles of Political Economy" and the theory of comparative advantage.
- (C) J.S. Mill → Author of "Principles of Political Economy" (1848).
Final Answer: \[\boxed{\text{John Maynard Keynes}}\]