The branch of economics that deal with human behavior and choices as they relate to relatively small units-an individual, a business, firm, or a single market.

Microeconomics.

Their concerns are based on effects of individual decisions.

The correct answer is B) a unit of account.

When money provides a standard way to compare the value of goods and services, it acts as a unit of account.

When acts as a union of account, money measure the exchange of value. Money represents a common denominator for the people so they can exchange products and services with money as the mean of value. The best expression for this to happen is the concept of price. When people know the value of something through its price, money act as the conduit for the exchange.