Step 1: Understand the liquidity trap.
A liquidity trap occurs when interest rates are so low that they cannot be reduced further to stimulate investment. In this situation, the demand for money becomes highly elastic, and increasing the money supply does not lead to increased investment or consumption.
Step 2: Analyze the options.
- Option (A) is incorrect because in a liquidity trap, monetary policy becomes ineffective as the economy is stuck with low interest rates and no incentive to borrow more.
- Option (B) is incorrect because hyperinflation is not typically associated with a liquidity trap.
- Option (C) is incorrect because interest rates cannot fall significantly in a liquidity trap, as they are already near zero.
- Option (D) is correct. In a liquidity trap, expansionary monetary policy, such as increasing the money supply, does not effectively stimulate the economy because the demand for money becomes perfectly elastic.
Final Answer:
\[
\boxed{\text{expansionary monetary policy is completely ineffective}}
\]