Correct Answer: (a) Marginal rate of substitution
Solution:The marginal rate of substitution (MRS) is the willingness of a consumer to replace one good for another good, as long as the new good is equally satisfying. When the law of diminishing MRS is in effect, the MRS forms a downward, negative sloping, convex curve showing more consumption of one good in place of another. Therefore, option (a) is the correct answer.