Final answer:
A perfectly inelastic demand curve is described as a vertical demand curve, and indicates that the quantity demanded remains constant, irrespective of price changes.
Step-by-step explanation:
A perfectly inelastic demand curve is best described as D. It is a vertical demand curve. This vertical demand curve indicates zero elasticity, meaning that the quantity demanded does not change regardless of the price level. Whether there is a price increase or decrease, the amount of the good demanded stays the same in the case of perfectly inelastic demand. In contrast, a perfectly elastic demand curve is a horizontal line, suggesting that an infinite quantity will be demanded at a specific price but the quantity demanded will drop to zero if the price changes even slightly.