Correct Answer: Diminishing marginal utility

Solution : The correct answer is (a) Diminishing marginal utility.

Indifference curves represent different combinations of goods or services that provide the consumer with the same level of satisfaction or utility. The concave shape of indifference curves is a reflection of the principle of diminishing marginal utility. According to this principle, as an individual consumes more of a particular good while holding the consumption of other goods constant, the additional utility or satisfaction derived from each additional unit of the good tends to diminish.

This diminishing marginal utility leads to the concave shape of indifference curves. As the consumer moves along an indifference curve from left to right, the quantity of one good increases while the quantity of the other good decreases. To maintain the same level of satisfaction, the consumer requires increasingly larger compensating quantities of the decreasing good to offset the reduction in the other good. This leads to the curve becoming steeper, resulting in a concave shape.