The correct answer is D. All of the above are correct.
When a market is in equilibrium, the quantity demanded of a good or service equals the quantity supplied. This means that the price is at a level where there is no shortage or surplus of the good or service. At this price, buyers and sellers are willing to trade the same amount of the good or service.
A shortage exists when the quantity demanded of a good or service is greater than the quantity supplied. This means that there are not enough goods or services available to meet the demand of buyers. As a result, the price of the good or service will tend to rise until the quantity demanded equals the quantity supplied.
A surplus exists when the quantity supplied of a good or service is greater than the quantity demanded. This means that there are more goods or services available than buyers are willing to purchase. As a result, the price of the good or service will tend to fall until the quantity demanded equals the quantity supplied.
In conclusion, when a market is in equilibrium, there is no shortage or surplus of the good or service. The price is at a level where buyers and sellers are willing to trade the same amount of the good or service.