Good X Is Measured on the Horizontal Axis and Good

Good X is measured on the horizontal axis and good Y is measured on the vertical axis.The marginal rate of substitution is

A) the relative price of good X in terms of good Y.
B) the relative price of good Y in terms of good X.
C) the rate at which a consumer will give up good Y to obtain an additional unit of good X while remaining indifferent.
D) the rate at which a consumer will give up good X to obtain an additional unit of good Y while remaining indifferent.
E) equal to the magnitude of the slope of the budget line.