Classical economists believe that real increases in aggregate demand (purchasing power) can only be caused by:

a) increases in productivity and technology.

b) increases in government spending.

c) increases in the money supply, as controlled by the Federal Reserve.

d) increases in the nation's national debt.

The correct option is a) increases in productivity and technology.

Explanation:

Classical economists always emphasize the use of invisible hands to stabilize the market and to regain the equilibrium itself. These economists are against government intervention in the economy. According to them, the real increase in aggregate demand may be the cause of an increase in technology and productivity of input resources. Increased productivity will lead to increase in the aggregate supply, which further leads to decrease in the price level. As a result, the purchasing power of the people increases.

All of the other options are based on macroeconomic policy either by the government or by Federal Reserve.