Final answer:
Box B on the circular flow model represents the product markets, where households pay for goods and services, providing revenue to firms, as indicated by Arrow "B".
Step-by-step explanation:
The part of the economy represented by box B on the circular flow model is the product markets. This is where households pay for goods and services, which in turn becomes the revenue for businesses or firms.
Arrow "B" in the circular flow model indicates the flow of money from households to the firms as they purchase goods and services. Arrows A and B together represent both sides of the product market: firms offering goods and services and households buying them.