The Marginal Rate of Substitution (MRS)of X for Y in Consumption

The marginal rate of substitution (MRS) of X for Y in consumption refers to the:

A) amount of X that a nation must give up for one extra unit of Y and still remain on the same indifference curve
B) amount of Y that a nation must give up for one extra unit of X and still remain on the same indifference curve
C) amount of X that a nation must give up for one extra unit of Y to reach a higher indifference curve
D) amount of Y that a nation must give up for one extra unit of X to reach a higher