What is an indifference curve convex to the origin?

Question

The law of ______________? I can't seem to remember the law of this answer. Will somebody help, Please?

Answer 1

A convex indifference curve implies that the Marginal Rate of Substitution (MRS) is decreasing. So I'm thinking you are referring to that but it it not exactly a law like the "law of diminishing returns"

Answer 2

It appoint to the laws of conduct of customer. There are four but I think it is referring preference revealed law