Concept:Perfectly inelastic demand is when the quantity demanded stays constant no matter how the price changes, giving zero price elasticity of demand.Explanation:Price elasticity of demand is calculated as Ed=%ΔP%ΔQd.When price changes but the quantity demanded remains unchanged, the change in quantity demanded is %ΔQd=0.So the elasticity becomes Ed=%ΔP0=0.This is called zero elasticity or perfectly inelastic demand.The demand curve for such a commodity is a vertical straight line.This occurs for essential goods with no close substitutes.Answer:D. Perfectly inelastic.