The concept being described in the question is the rate at which a consumer is willing to substitute one good for another while keeping the same level of satisfaction. This is known as the marginal rate of substitution.

Option 1: Marginal rate of substitution. This option correctly identifies the concept described in the question. It refers to the rate at which a consumer is willing to give up one unit of a good in order to obtain an additional unit of another good, while maintaining the same level of satisfaction. The magnitude of the marginal rate of substitution indicates how much of one good a consumer is willing to give up for a unit of another good.

Option 2: Marginal rate of technical substitution. This option refers to the rate at which a firm is willing to substitute one input for another in the production process, in order to maintain the same level of output. It is not the concept being described in the question.

Option 3: Diminishing marginal utility. This option refers to the concept that as a consumer consumes more of a good, the additional utility or satisfaction derived from each additional unit decreases. While related to consumer choice, it is not the specific concept being described in the question.

Option 4: Equi-marginal utility. This option refers to the concept of