Final answer:
Price Elasticity of Demand measures the responsiveness of quantity demanded to price changes. It is calculated using (ΔQ/Q)/(ΔP/P)
Step-by-step explanation:
The subject of this question is Price Elasticity of Demand. It is a concept in economics that measures the responsiveness or sensitivity of the quantity demanded of a good or service to changes in its price. It is calculated using the formula:
Price Elasticity of Demand = (ΔQ/Q)/(ΔP/P)
The options provided in the question, namely (a) Inelastic Demand, (b) Perfectly Elastic or horizontal Demand, (c) ΔQd, and (d) (ΔQ/Q)/(ΔP/P), are all related to understanding and analyzing price elasticity of demand. In particular, option (b) refers to a perfect or infinitely elastic demand curve where any price change leads to an infinite change in quantity demanded.