Answer: Setting the interest rate to zero

## Explanation When interest rate models forecast negative short-term interest rates, the most appropriate and commonly used approach is to **set the interest rate to zero** (Option C). Here's why: ### Why Option C is Correct: 1. **Economic Reality**: Negative interest rates are theoretically possible but practically problematic in many markets. Setting rates to zero creates a lower bound that reflects the economic reality that nominal interest rates cannot go significantly negative. 2. **Model Stability**: This approach prevents the model from generating unrealistic negative values that could distort option pricing calculations. 3. **Industry Standard**: Many financial institutions and risk management frameworks use zero floors in interest rate models to handle this issue. ### Why Other Options Are Incorrect: - **Option A (Adjusting risk-neutral probabilities)**: This would fundamentally change the pricing framework and could introduce arbitrage opportunities. - **Option B (Increasing volatility)**: This would exacerbate the problem by making extreme negative rates more likely. - **Option D (Setting mean-reverting parameter to 1)**: While mean reversion helps, setting it exactly to 1 doesn't guarantee prevention of negative rates and may not be the most direct solution. ### Additional Context: In interest rate modeling, particularly with models like the Cox-Ingersoll-Ross (CIR) model or Black-Karasinski model, practitioners often implement **zero floors** or **non-negative constraints** to prevent negative interest rate scenarios that are economically unrealistic in many contexts.

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