A consumer's total utility level being the same, the amount of good 'X' that the consumer has to forego, in order to get an additional unit of good 'Y' is termed as :

Marginal Rate of substitution

The correct answer is option (2) : Marginal Rate of substitution

The "Marginal Rate of Substitution" (MRS) is a concept in economics that measures the rate at which a consumer is willing to exchange one good (in this case, good 'X') for another good (good 'Y') while keeping their total utility constant. The MRS represents the consumer 's preferences and how they are willing to make trade-offs between goods. When a consumer is at a point where their utility remains the same, they are essentially making a trade-off between consuming more of one good (Y) at the expense of consuming less of the other good (X). The MRS quantifies the rate at which the consumer is willing to make this exchange.

If the consumer has to forego a small amount of good 'X' to gain an additional unit of good 'Y' while keeping total utility constant, the MRS is the slope of the indifference curve at that point. It measures the consumer's willingness to give up some of one good in exchange for more of the other.