Final answer:
Box B on the circular flow model represents the product markets where households pay for goods and services, effectively highlighting the exchange between businesses and households.
Step-by-step explanation:
The part of the economy represented by box B on the circular flow model is the product markets. In the circular flow model, firms produce and sell goods and services to households, which is indicated by arrow A. Conversely, households pay for goods and services, which become the revenues for firms, and this flow of money from households to firms through the product market is indicated by arrow B. Hence, arrows A and B represent the two sides of the product market - the business side where firms offer goods and services, and the household side where payment is exchanged for these goods and services.