Final answer:
President Franklin D. Roosevelt applied the Keynesian economic idea of increasing government spending during times of high unemployment to help the United States recover from the Great Depression, particularly through programs like the WPA.
Step-by-step explanation:
The idea from Keynesian economics that President Franklin D. Roosevelt put into practice to help the nation recover from the Great Depression was C. Government should increase public spending in times of high unemployment. Roosevelt implemented this Keynesian economic theory through large-scale government spending, particularly in public works and job relief programs like the Works Progress Administration (WPA). This led to deficit spending to stimulate the economy and, as Keynesian economists argued, it was a necessary measure to revive employment and consumer spending. These actions helped pull the United States out of the depths of the Great Depression and set the stage for subsequent economic recovery during World War II.