What causes an indifference curve to be convex to the origin?

The correct answer is Option (3) → Law of Diminishing Marginal Rate of Substitution

An indifference curve shows different combinations of two goods that give the consumer the same level of satisfaction. It is convex to the origin because of the Law of Diminishing Marginal Rate of Substitution (MRS), which states that: As a consumer substitutes one good (say X) for another (say Y), the amount of Y the consumer is willing to give up for each additional unit of X decreases, while maintaining the same satisfaction level.

This declining willingness to substitute causes the curve to bend inward (convex) toward the origin.