Answer: A short American call option on the underlying pool of mortgages.

## Explanation From an investor's perspective, a mortgage-backed security (MBS) can be decomposed into: - **Long position in a non-prepayable mortgage pool**: This represents the basic cash flows from the mortgages without the prepayment feature - **Short American call option on the underlying pool of mortgages**: This represents the prepayment risk that investors face ### Why this is correct: 1. **Prepayment as a call option**: When interest rates fall, homeowners have the right (but not the obligation) to prepay their mortgages. This is economically equivalent to them holding a call option on their mortgage debt. 2. **Investor's perspective**: From the investor's viewpoint, they are effectively writing (selling) this call option to the homeowners. When homeowners exercise their prepayment option, investors lose the higher-yielding mortgages and must reinvest at lower rates. 3. **American-style option**: The prepayment option can be exercised at any time (not just at maturity), making it an American-style option. 4. **Economic impact**: The short call position explains why MBS investors face negative convexity - when rates fall, the value of MBS doesn't rise as much as non-callable bonds due to the increased likelihood of prepayment. This decomposition helps explain the risk-return characteristics of mortgage-backed securities and why they trade at higher yields than comparable Treasury securities.

Author: LeetQuiz .