• The correct answer is a. the periodic fluctuation of economic activity.

The concept known as the business cycle is used in economics to illustrate the downward and upward movements of the economy of a country. The business cycle represents the GDP fluctuations of an economy over time and it includes four main stages of an economy which are expansion, peak, recession, and trough. Therefore, the correct answer is option a. In contrast, option b. is incorrect because that concept is not associated with the business cycle. Option c. is also incorrect because the business cycle does not have a specific timeframe. Option d. is also incorrect because the life cycle theory is related to the income and decision making of households.