Final answer:
Income is the correct answer as it refers to the flow of earnings over time, which can come from various sources such as employment, business activities, or investments.
Step-by-step explanation:
The answer to the question income is a flow of earnings per unit of time is A) Income. Income represents the money that people or businesses receive over a certain period, often measured on an annual basis. This can include wages from employment, profits from business activities, or investment income. It's important to note that income differs from other financial terms such as wealth, which refers to a country's total accumulated wealth, or currency, which is a system of money in general use in a particular country.