The correct answer is: B. Budget constraints.
A budget constraint is a line that shows the combinations of goods and services that a consumer can afford to buy with a given amount of money. It is determined by the consumer’s income and the prices of the goods and services.
A consumer’s spending is restricted because of the budget constraint. The consumer can only buy the combinations of goods and services that lie on or below the budget constraint.
Option A, utility maximization, is incorrect because it is the goal of the consumer. The consumer wants to maximize their utility, but they are constrained by their budget.
Option C, demand curve, is incorrect because it is a graph that shows the relationship between the price of a good and the quantity demanded of that good. The budget constraint is not a graph.
Option D, marginal utility, is incorrect because it is the additional satisfaction that a consumer gets from consuming one more unit of a good. The budget constraint is not related to marginal utility.