Final answer:
The money supply curve is vertical because we assume the central bank controls the money supply.
Step-by-step explanation:
The correct answer is d. vertical because we assume the central bank controls the money supply.
The money supply curve represents the relationship between the quantity of money supplied and the value of money.
In this case, if the central bank controls the money supply, the vertical position of the curve indicates that the quantity of money supplied remains constant regardless of changes in the value of money.
For example, if the value of money decreases, people may demand more money to purchase goods and services.
However, the central bank can counteract this increase in demand by reducing the money supply, keeping the quantity of money supplied constant, resulting in a vertical money supply curve.