The correct statement is option (D) Demand is the quantity of a product that people are willing and able to purchase at different prices.
We know that demand means the number of goods people want to consume given the prices of the good.
In option (A) Demand is the total quantity of a product that people are willing, even if unable, to purchase at a given price, the last part that even if unable is wrong because people will demand only that much that they can purchase.
In option (b) Demand for a product is the same as the quantity demanded of a product, we get that this is wrong as quantity demanded only is influenced by price but demand can also be influenced by other factors.
The option (C) Demand represents the different quantities of a good or service that provides consumers the same amount of utility, is not the definition of demand but indifference curve.
Therefore, the correct statement is option (D) Demand is the quantity of a product that people are willing and able to purchase at different prices.
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