Final answer:
The money supply equals $800 billion.
Step-by-step explanation:
To calculate the money supply, we multiply the monetary base by the money multiplier. In this case, the monetary base is $400 billion and the money multiplier is 2.
Money supply = Monetary base * Money multiplier
Money supply = $400 billion * 2 = $800 billion
Therefore, the correct answer is c. $800 billion.