Which factor directly affects the quantity demanded of a product?

The quantity demanded of a product is directly influenced by its price. This relationship is described by the law of demand, which states that all else being equal, as the price of a product rises, the quantity demanded typically falls, and conversely, as the price decreases, the quantity demanded tends to increase. This negative relationship between price and quantity demanded is due to two main effects: the substitution effect, where consumers switch to alternative products when prices rise, and the income effect, where a change in price affects consumers' real income and their purchasing power.

In contrast, factors like production technology, the number of sellers, and the state of the economy can influence demand indirectly but do not determine quantity demanded directly at a given price level. For example, improved production technology might lead to lower prices, which could increase quantity demanded, but it is the price change itself that has the direct effect.