In cycle theory, fluctuations in both investment and consumption expenditure, driven by fluctuations in the growth rate of the quantity of money, are the main source of fluctuations in aggregate demand.

Monetarist

Explanation:

The monetarist theory is a theory which believes that the changes in the supply of money is the most important factor in the growth of an economy.

In this concept, economic stability can be fostered through targeting the money supply. The theory assume that the fluctuations in both the investment and consumption expenditure, which are s a result of the fluctuations in growth rate of the quantity of money, are the main source of fluctuations in aggregate demand.

b is the correct answer