The correct answer is D.
Price elasticity of demand measures the responsiveness of consumers to a change in price. It is calculated by dividing the percentage change in quantity demanded by the percentage change in price. A high price elasticity of demand means that consumers are very sensitive to price changes, and a low price elasticity of demand means that consumers are not very sensitive to price changes.
Option A is incorrect because the slope of the demand curve measures the responsiveness of quantity demanded to a change in income.
Option B is incorrect because the number of buyers in a market does not affect the price elasticity of demand.
Option C is incorrect because the extent to which the demand curve shifts as the result of a price decline is determined by the price elasticity of demand.