Answer: The correct answers are "B. flow; stock" and "C. purchase of new plants and equipment by firms and the purchase of new houses by households."
Explanation: Income is a flow variable, and financial wealth is a stock variable. The term investment, as used by economists, refers to the purchase of new plants and equipment by firms and the purchase of new houses by households.
Flow: variable whose quantity is measured per unit or given period of time; for example, income, investment.
Stock: variable whose quantity is measured at a certain moment of time; for example: population, wealth, capital stock.
The investment is related to using resources in order to achieve some benefit.