The correct answer is C. Business cycles.
Business cycles are the fluctuations in economic activity over time. They are characterized by alternating periods of expansion and contraction. Expansions are periods of economic growth, when businesses are investing and hiring, and unemployment is low. Contractions are periods of economic decline, when businesses are cutting back on investment and hiring, and unemployment is high.
Monetary policy is the use of interest rates and other tools to control the money supply. Fiscal policy is the use of government spending and taxation to influence the economy. Neither of these policies is directly related to the study of business cycles.
The study of business cycles is important because it can help us to understand the causes of economic fluctuations and to develop policies to mitigate their effects.