The correct answer is: B. Law of demand.
Diminishing marginal utility is the principle that as a consumer acquires more of a good, the additional satisfaction (or utility) gained from each additional unit of the good decreases. This is because as a consumer acquires more of a good, they become less and less hungry for it.
The law of demand states that, all other things being equal, the quantity demanded of a good will decrease as the price of the good increases. This is because as the price of a good increases, consumers will be less willing to purchase it.
Diminishing marginal utility is the basis of the law of demand because it explains why consumers are willing to pay less for additional units of a good as they acquire more of it.
The law of supply states that, all other things being equal, the quantity supplied of a good will increase as the price of the good increases. This is because as the price of a good increases, producers will be more willing to produce it.
The law of returns states that, in the short run, as a firm increases the amount of one input it uses, while holding all other inputs constant, the output of the firm will eventually increase at a decreasing rate. This is because as a firm increases the amount of one input it uses, it will eventually reach a point where the additional output it gains from each additional unit of the input is less than the additional output it gained from the previous unit of the input.
Therefore, the correct answer is: B. Law of demand.