Answer:
The answer is: D) Both a and b are correct.
Explanation:
The marginal rate of substitution (MRS) measures the units of good A that a consumer is willing to give up for another unit of good B. It works with goods that are equally satisfying.
The MRS decreases along the indifference curve if it is convex-shaped (law of diminishing marginal rates of substitution), if the MRS is constant the curve will be a straight line, if the MRS increases the curve will be concave-shaped.