Which of the following is NOT a main function of money?

The function that distinguishes itself as not being a central role of money is the concept of "source of labor." Money primarily serves three main functions within an economy: as a unit of account, a store of value, and a medium of exchange.

The role of money as a unit of account means it provides a standard measurement for pricing goods and services, allowing for a consistent way to compare values and keep track of economic transactions. As a store of value, money retains its purchasing power over time, enabling individuals to save and plan for future expenditures. When acting as a medium of exchange, money facilitates transactions by eliminating the inefficiencies of barter, allowing for goods and services to be exchanged more easily and efficiently.

In contrast, "source of labor" refers to the availability and capability of individuals to work, which is related to labor markets and economic production, not a function of money itself. This distinction clarifies why the correct answer is not considered a main function of money.