Answer:
The correct answer is letter "D": the interest rate on bonds.
Step-by-step explanation:
Investors tend to keep their money liquid as cash, and expect attractive returns for sacrificing their liquidity, according to the Liquidity Preference Theory. Investors will pay more for short-term debt and the liquidity that comes with it, and they will also seek higher interest rates before taking on longer-term debt.
Thus, the opportunity cost of holding money, according to the liquidity preference theory would be the returns obtained from investing in assets such as stocks or interest rates with bonds.