The correct answer is A. Utility theory.
Utility theory is a branch of microeconomic theory that studies the relationship between the satisfaction (utility) consumers receive from consuming goods and services and the prices they pay for them. It is used to explain how consumers make choices about what to buy.
Indifference curve analysis is a tool used in utility theory to show the combinations of goods and services that a consumer is indifferent between. An indifference curve is a curve that shows all the combinations of goods and services that a consumer would be equally happy with.
Revealed preference theory is a theory that uses a consumer’s purchasing decisions to infer their preferences. It is based on the idea that consumers will only buy goods and services that they prefer to other goods and services that they could have bought with the same amount of money.
Inferior goods are goods that consumers demand less of as their income increases. This is because as consumers’ incomes increase, they are able to afford to buy more of the goods that they prefer, and inferior goods are no longer as attractive to them.
In conclusion, the correct answer is A. Utility theory.