The AD Curve . a. all of the choices are correct.
b. is downward sloping, because with higher inflation comes higher interest rates and lower spending, so equilibrium aggregate output declines
c. indicates the level of aggregate output corresponding to different goods-market-clearing levels of the interest rate
d. explains how inflation affects output in the long run

a. all of the choices are correct.

Explanation:

In Economics, there are primarily two (2) factors which affect the availability and the price at which goods and services are sold or provided, these are demand and supply.

Demand can be defined as the total amount of goods or services that consumers are willing and able to purchase at a given price.

The Law of Demand states that quantity of goods or services demanded varies inversely with their price, other things constant or being equal. Thus, the smaller the quantity of goods or services demanded, the higher the price and vice-versa.

Aggregate demand (AD) can be defined as the total quantity of output (final goods and services) that is demanded by consumers at all possible price levels in an economy at a particular time.

An aggregate demand (AD) curve gives a negative relationship between the aggregate price level for goods or services and the quantity of aggregate output demanded in an economy at a specific period of time.

In conclusion, all of the aforementioned statements best describes an aggregate demand (AD) curve.