Final answer:
We use price elasticity of demand to compare how the quantity demanded responds to a change in price across different markets. It is a measure calculated by dividing the percentage change in quantity demanded by the percentage change in price.
Step-by-step explanation:
We compare the responsiveness of quantity demanded to a change in price across markets using the concept of price elasticity of demand. This is calculated by dividing the percentage change in quantity demanded by the percentage change in price. Price elasticity can be described as elastic, unit elastic, or inelastic. Elastic demand indicates that quantity demanded is highly responsive to price changes, whereas inelastic demand indicates that quantity demanded is not very responsive to price changes. A unitary elasticity means the percentage change in price and quantity demanded are equal. Therefore, to answer the student's question, the correct option is (a) Price elasticity of demand.