Economists define investment to include

A) any increase in business inventories.
B) the addition of cash to a savings account.
C) the purchase of common or preferred stock.
D) the purchase of any durable good,e.g. ,an automobile or a refrigerator.

Investment, in economic terms, includes any increase in business inventories, as this represents goods produced but not yet sold, indicating a commitment of resources for future sales. Choices B, C, and D are not considered investments in the economic sense; adding cash to savings is saving, not investing, purchasing stocks is a financial investment but not in the economic sense of producing goods or services, and buying durable goods is consumption.