Final answer:
Box b in the circular flow model represents businesses, which demand resources from households in the resource market and sell goods and services in the product market. Option 3.
Step-by-step explanation:
The part of the economy represented by box b in the circular flow model is businesses(3). These entities are crucial within the economy as they are involved in the production of goods and services. Within the circular flow model, businesses interact with households in two specific ways: they demand resources such as labor from households in the resource market, and they offer goods and services to households in the product market. The resources they acquire from households are utilized to create products and services, which are in turn sold back to households, fueling the continuous flow of economic activity.