The correct answer is D.
The marginal rate of substitution (MRS) is the rate at which a consumer is willing to trade one good for another. It is measured as the amount of one good that a consumer is willing to give up to obtain one more unit of another good, while remaining at the same level of utility.
An indifference curve is a curve that shows all the combinations of goods that a consumer is indifferent between. In other words, all the points on an indifference curve represent the same level of utility for the consumer.
If the MRS is 3, then the consumer is willing to give up three units of the good measured along the Y-axis for one unit of the good measured along the X-axis. This means that the consumer values the good measured along the X-axis three times more than the good measured along the Y-axis.
Option A is incorrect because the consumer is not willing to pay for the good measured along the Y-axis. Option B is incorrect because the consumer is not willing to give up one unit of the good measured along the Y-axis for three units of the good measured along the X-axis. Option C is incorrect because the consumer is not willing to give up three units of the good measured along the Y-axis for one unit of income.