Final answer:
A consumer opting for a luxury car over an economy car is making a choice based on quality differentiation, (option a) which corresponds to purchasing normal goods with a rise in income.
Step-by-step explanation:
A consumer who chooses to buy a luxury car rather than an economy car is responding to product differentiation by quality. When individuals have higher incomes, they tend to demand more normal goods, which are goods that see increased demand as income rises. Luxury cars are an example of a normal good, as they are often more desirable and purchased more frequently by those with higher incomes. This consumer behavior reflects a rational choice to spend income on higher quality items that are preferred, assuming the consumer has the discretionary income to support such a decision. It does not constitute a violation of the law of demand, which states that quantity demanded of a good falls as the price of the good rises, ceteris paribus.