What is the Marginal Rate of Substitution (MRS)?

(A) The rate at which a consumer is willing to substitute one good for another.
(B) Equal to the slope of the indifference curve.
(C) Changes as we move along the indifference curve.
(D) Is constant for perfect substitutes.

Choose the correct answer from the options given below:

(A), (B) and (D) only

The correct answer is Option (1) → (A), (B) and (D) only

Note: The given answer is as per NTA. however, there appears to be a mistake in the answer. All the given statements are correct and the answer should be Option 3- (A), (B), (C) and (D) as explained below:

The Marginal Rate of Substitution (MRS) is a fundamental concept in consumer theory, representing how a consumer trades off one good for another while maintaining the same level of satisfaction. It's depicted by the slope of an indifference curve.

(A) The rate at which a consumer is willing to substitute one good for another. Correct. This is the core definition of MRS. It quantifies how much of good Y a consumer is willing to give up to get an additional unit of good X, without changing their overall utility.

(B) Equal to the slope of the indifference curve. Correct. An indifference curve plots all combinations of two goods that provide a consumer with an equal level of utility. The slope of this curve at any point reveals the MRS between the two goods at that specific combination. Geometrically, it's the negative of the slope of the indifference curve.

(C) Changes as we move along the indifference curve. Correct. For standard consumer preferences, the indifference curve is convex to the origin due to the Law of Diminishing Marginal Rate of Substitution. As a consumer moves downward along the curve from left to right, they have more of Good X and less of Good Y, making them less willing to give up Y for more X. Thus, the MRS continuously decreases (changes) along the curve. Thus, the Statement (C) is generally true for convex indifference curves where diminishing marginal rate of substitution operates. In the special case of perfect substitutes, however, the MRS remains constant, as stated in (D).

(D) Is constant for perfect substitutes. Correct. Perfect substitutes are goods that a consumer considers to be identical or interchangeable (e.g., different brands of generic painkillers if they have the same active ingredient). For perfect substitutes, the consumer is willing to trade them at a constant rate, regardless of how much of each they possess. This results in straight-line indifference curves, and thus, a constant MRS along the indifference curve.