Explanation

When dealing with negative interest rates in interest rate models, the most practical and commonly used approach is:

• Option D: Setting negative rates to zero is the most appropriate action. This approach recognizes that while negative rates can occur in models, in practice, rates typically have a lower bound of zero (or slightly negative in some modern contexts). Setting negative rates to zero prevents unrealistic scenarios and maintains model stability.

Let's evaluate the other options:

• Option A: Using a normal distribution actually increases the likelihood of negative rates, as normal distributions have unbounded support.
• Option B: Adjusting risk-neutral probabilities is complex and may not directly address the negative rate issue.
• Option C: Increasing volatility when rates are negative would exacerbate the problem rather than solve it.

Therefore, option D represents the best practice for handling negative interest rates in pricing models.