A downward-sloping demand curve in economics is primarily explained by the notion of **(3) Diminishing Marginal Utility**.

Here's an explanation for why this is the case:

1. Diminishing Marginal Utility (DMU) is a fundamental concept in economics that states that as an individual consumes more units of a particular good or service, the additional satisfaction or benefit (utility) derived from each additional unit consumed decreases. In other words, the more you consume of a product, the less you value each additional unit.
2. This concept is closely related to the law of demand, which states that, all else being equal, as the price of a good decreases, the quantity demanded for that good increases, and vice versa. When we combine the idea of diminishing marginal utility with the law of demand, we can understand why a downward-sloping demand curve exists:
3.  At higher prices, consumers buy fewer units of a product because the marginal utility of each additional unit is lower than the price they would have to pay.3
4. . As the price decreases, the marginal utility of each additional unit becomes relatively higher, making consumers willing to buy more units at the lower price.

In this way, the downward-sloping demand curve illustrates the inverse relationship between price and quantity demanded, driven by the concept of diminishing marginal utility.