What event was a major influence on the development of macroeconomics? a

Great Depression

Great Depression lasted from 1929 to 1939 and severly impacted world's economy, the world economy was in decline, the unemployment was on the rise, stock markets crashed, banks failed and the whole world was impacted. So, definitely the Great Depression affected the economy but did not contributed in its development.

The Employment Act of 1946

The Employment Act of 1946 was supposed to maintain the high level of employment, increase production and maintain high level of output but failed to impact the economy majorly.

Establishment of Federal Reserve

Federal Reserve was established in the year 1913 and since then has been working as the Central Bank of United States. Before Federal Reserve there free banking era where there was very little regulation on the banks and in the absence of central banking system, many banks failed. Creation of Federal Reserve brought stability in the banking System. Now the Fed looks after the inflation and interest rate and decide its policies keeping the macroeconomics factor in mind.

The US entry in World War II

The US entry also impacted the economy as the soldiers were sent to war, war equipment were produced at large scale and the employment increased during the war. However after the war ended and the production was at normal levels the employment level was gradually decreasing for which the employment act was passed.

From the above discussion it is clear that Creation of Federe Reserve System was a major influence on the development of macroeconomics

So the answer is (c)