Final answer:
The study identified four categories of demand: consumer spending (consumption), business spending (investment), government spending, and external sector spending (net exports).
Step-by-step explanation:
The four categories of customers or demand in the economy, as outlined in the study presented in the question, include consumer spending (also known as consumption), business spending (investment), government spending on goods and services, and spending by the external sector (net exports).
Firms in the goods market, firms in the financial market, households in the goods market, and households in the financial market are not accurate answers to this question as they are representatives of different segments in the economy that interact with each other rather than categories of customers or types of demand.