Option 1: Zero -> When demand remains constant regardless of price changes, the percentage change in quantity demanded is zero. Since elasticity of demand = (% change in quantity demanded) / (% change in price), and the numerator is zero, the elasticity equals zero. This is called perfectly inelastic demand, represented by a vertical demand curve. Examples include essential medicines with no substitutes or life-saving drugs where consumers must buy the same quantity regardless of price. -> correct