What typically causes a decrease in equilibrium quantity while equilibrium price remains unchanged?

A decrease in equilibrium quantity with an unchanged equilibrium price is typically caused by a decrease in supply. When supply decreases, the quantity of a good or service that producers are willing and able to sell at a given price level falls. However, if demand remains constant, this can lead to a situation where the market reaches a new equilibrium quantity that is lower than before. The equilibrium price, in this case, can remain unchanged if there is a relatively inelastic demand. This means that while suppliers are less willing to produce at the same quantity, the demand remains unchanged and steady at the previous price, hence the equilibrium price does not shift.

In the context of the other options, an increase in demand would generally lead to an increase in both equilibrium price and quantity. A shift in consumer preferences could alter demand patterns, likely impacting both price and quantity depending on the nature of that shift. A price ceiling, which is a maximum limit set by the government on how high a price can go, could also lead to a surplus or shortage, thereby affecting equilibrium quantity, but it does not explain why both equilibrium price and quantity would change in opposite directions without additional context.