The marginal rate of substitution (MRS) MRS is defined as the number of units of one good (namely Y) that must be given up if the consumer, after receiving an extra unit of another good (namely X) , is to maintain a constant level of satisfaction. In other words, the marginal rate of substitution is the amount of a good that a consumer is willing to give up for another good, as long as it is equally satisfying. The correct answer is A.