The correct answer is: A. a-1, b-2, c-4, d-3
Marginal rate of substitution (MRS) is the rate at which a consumer is willing to give up one good in exchange for another. It is measured as the ratio of the marginal utility of the two goods.
A consumer’s indifference curve (IC) is a graph that shows all the combinations of goods that give the consumer the same level of satisfaction. The slope of an IC is equal to the MRS.
If MRS is zero, then the consumer is indifferent between the two goods. This means that the consumer is not willing to give up any of one good in exchange for the other. This is the case when the consumer is consuming the two goods in the same proportions.
If MRS is constant but non-zero, then the consumer is willing to give up the same amount of one good in exchange for the other. This means that the consumer is indifferent between all the combinations of goods that lie on a straight line with a negative slope.
If MRS is decreasing, then the consumer is willing to give up less of one good in exchange for the other as they consume more of the other good. This means that the consumer is getting more satisfaction from the other good as they consume more of it. This is the case when the IC is concave to the origin.
If MRS is increasing, then the consumer is willing to give up more of one good in exchange for the other as they consume more of the other good. This means that the consumer is getting less satisfaction from the other good as they consume more of it. This is the case when the IC is convex to the origin.