The concept of demand is best described as

a. the total satisfaction that consuming a good provides people at different prices.
b. the quantity of a good or a service that people will offer for sale at different possible prices.
c. the additional satisfaction derived from a quantity of goods and services obtained when income increases.
d. the quantity of a good or a service that people are willing and able to purchase at different possible prices.
e. the quantity of a good or service that consumers will substitute when the price of a good changes

Demand can be defined as an economic principle that corresponds to the quantity of a good or service that consumers want to purchase according to a price defined in a market in relation to a period of time.

The fundamental objective of the economy is to satisfy the needs of individuals through goods and services, such as shelter, food, health, clothing, leisure, etc. Therefore, demand reflects the consumer's demand for these goods and services and their price justifies the satisfaction of consuming this good.

Demand depends on two factors:

1- Usefulness of satisfaction of desire or need

2- The consumer's ability to pay for the good or service

One of the main theories of microeconomics is the demand theory. Through it it is possible to identify what and how much people want to consume goods and services and the impact that income and satisfaction levels affect demand.