if the price of a commodity Z falls and a consumer buys less of it, then commodity Z is a

Analyze the question: The price of commodity Z falls, and the consumer buys less of it. This is the opposite of the normal law of demand, which says that when price falls, quantity demanded rises.

Explain the concept:

• Necessity / normal good: These follow the law of demand. A fall in price leads to a rise in quantity demanded.
• Good of ostentation (Veblen good): People buy these for prestige. A higher price makes them more desirable, so demand rises when price rises. The reverse also holds: if the price falls, the good loses its prestige appeal and people buy less. This is a possible contender here.
• Giffen good: This is a strongly inferior good that takes up a large part of a poor consumer's budget. When its price falls, the consumer's real income rises, so they switch to better alternatives and buy less of the Giffen good. The income effect outweighs the substitution effect, so the demand curve slopes upward.

Solution:

1. Price of Z falls and quantity demanded falls, so the law of demand is violated.
2. This rules out A (necessity) and C (normal good), which obey the law of demand.
3. That leaves B and D. Both show an upward-sloping demand curve.
4. The standard textbook definition of a Giffen good is "an inferior good whose quantity demanded falls when its price falls (and rises when its price rises)". This matches the question exactly.
5. A good of ostentation is usually described in terms of a price rise making the good more attractive for status. The question is phrased in terms of a general fall in price and reduced purchases, which is the classic Giffen description.

Tip: Remember the pairing: Giffen = inferior good + price falls → buy less (income effect beats substitution effect). For ostentation goods, think "prestige": the price rise makes the good more desirable. If the question says "falls and buys less" in the context of a poor consumer or an inferior good, choose Giffen.