An explanation for a downward sloping demand curve rests on the notion of :

Diminishing Marginal Utility

The correct answer is option (3) : Diminishing Marginal Utility

Diminishing Marginal Utility: As a consumer consumes more units of a good, the additional satisfaction (marginal utility) gained from consuming each additional unit decreases. Because of this, consumers are willing to pay less for additional units, resulting in a downward-sloping demand curve. This concept is a fundamental principle in microeconomics explaining why demand curves slope downwards from left to right.