Because successive units of a good produce less and less additional satisfaction, the price must fall to encourage a buyer to purchase more units of the good. This statement is most consistent with which explanation for the law of demand?
A) The rationing function of prices
B) The substitution effect
C) Diminishing marginal utility
D) The income effect

Economics explains that as a person consumes one more unit of a particular good or service, the value they associate to this falls. This is to state that consumption of each additional unit of a good diminishes the value and such the utility an individual derives from consumption. This follows that in order to make additional units viable and make consumers want more additional units of a product, the price needs to be reduced which then helps to either raise or stabilize utility that a consumer derives.

This is also why the consumer demand curves are downward sloping.

Hope this helps.