What results in a decrease in equilibrium quantity for a product?

A decrease in equilibrium quantity for a product occurs when there is a shift in either the demand or supply curve that leads to lower quantities being bought and sold. When both demand and supply decrease, the equilibrium quantity will drop because there are fewer goods being demanded at every price level while simultaneously fewer products are available for sale. This dual impact directly results in a lower quantity exchanged in the market.

For example, if consumer demand drops due to changing tastes or preferences away from the good, that would decrease demand, leading to fewer products being bought. Meanwhile, if producers anticipate this decline or face rising production costs, they might reduce the quantity they supply to the market. The combination of these two factors effectively lowers the equilibrium quantity.

In scenarios involving just a decrease in supply or a change in consumer preferences alone, the outcomes for equilibrium quantity could vary based on additional market factors and might not necessarily result in a decrease. For instance, a decrease in supply usually increases price, which could lead to a decrease in quantity demanded but doesn't always guarantee a net reduction in total quantity exchanged.

Thus, the choice indicating a decrease in both demand and supply is correct in identifying the conditions necessary for a decrease in equilibrium quantity.