Final answer:
The factor that changes the quantity demanded (Qd) or quantity supplied (Qs) of a product is price; however, advertising can significantly influence demand by either making the demand curve more inelastic or shifting it to the right, thus increasing demand.
Step-by-step explanation:
When considering the factors that affect the quantity demanded (Qd) or quantity supplied (Qs) of a product, price is the key factor that inherently causes movements along the demand and supply curves. This means an increase in price typically lowers the quantity demanded, as per the Law of Demand, and increases the quantity supplied, following the Law of Supply. However, in the context of this discussion, advertising can significantly impact the demand side by either making the demand curve more inelastic or by shifting the demand curve to the right, indicating an increase in product demand. By differentiating a product from its competition, advertising can thus increase demand, allowing firms to sell more or at higher prices.
It's important to note that while supply can change due to various factors like production costs, technology, and number of sellers, changes in demand can also result from shifts in consumer tastes, income, expectations, and, notably, the effects of advertising. When a successful advertising campaign differentiates a product, it alters consumer perception and, consequently, increases the product's demand.