Final answer:
Elasticity is a measure of the responsiveness of quantity demanded to price changes and is calculated by dividing the percentage change in quantity by the percentage change in price.
Step-by-step explanation:
Elasticity is a units-free measure of the responsiveness of the quantity demanded of a good or service to a change in its price. It is a percentage change in quantity demanded (or supplied) divided by the percentage change in price. Option C is correct because elasticity allows us to compare the responsiveness of the same good when quantity is measured in different units, or we can compare across different goods.