Keynes's criticism of the classical theory was that the Great Depression would not correct itself. The multiplier effect would restore an economy to full employment if a. government would follow a "least government is the best government" policy. b. government taxes were increased. c. government spending were increased. d. government spending were decreased.

According to Keynes, the classical theory failed to understand the severity of the Great Depression and believed that the economy would self-correct. However, he argued that a significant boost in economic demand was necessary to restore the economy to full employment. The multiplier effect is a concept in Keynes's theory, which states that a small increase in government spending can create a large increase in national income and employment.

We need to analyze each option and determine its impact on the economy according to Keynes's theory. a. "Least government is the best government" policy: This policy advocates for minimal government interference and would not actively address low demand. b. Increase in government taxes: Raising taxes would decrease disposable income, which could further lower demand and slow down the recovery. c. Increase in government spending: This policy aligns with the multiplier effect. By increasing government spending, demand would also increase, leading to higher national income and employment. d. Decrease in government spending: This policy would have the opposite effect of what Keynes argued. Lower government spending would reduce demand and hinder the economic recovery.

Based on our analysis of the given options and Keynes's criticism of the classical theory, we can infer that the correct policy to restore an economy to full employment using the multiplier effect is: c. government spending were increased.