If a consumer is at equilibrium while consuming two goods, X and Y, where an indifference curve intersects their budget line, which of the following conditions must hold true for maximizing utility?

When a consumer is at equilibrium, the chosen bundle of goods X and Y must lie on the budget line and on the highest attainable indifference curve, so the marginal rate of substitution (MRS) between X and Y equals the ratio of their prices. In other words, the slope of the indifference curve, which is the MRS, must match the slope of the budget line, which is the price ratio Px/Py. This ensures that the consumer cannot increase utility by moving along the budget line. For example, if the price of X is twice the price of Y, the consumer will only be satisfied if the trade‑off between X and Y in the indifference curve is also two to one. Thus, the condition MRS = Px/Py is necessary for utility maximization.