What is generally the least volatile interest rate?

The passbook savings rate is generally regarded as the least volatile interest rate due to its inherent characteristics. Passbook savings accounts offer a fixed interest rate that is set by the financial institution and is typically not subject to market fluctuations in the same way that other rates are. These accounts provide liquidity and allow for easy access to funds, while offering a relatively stable return on savings.

In contrast, fixed-rate mortgages, although they have a fixed rate for the duration of the loan, can still be impacted by wider interest rate trends, especially when viewed over the long term. Certificate of Deposit (CD) rates, while also generally stable and providing fixed returns for specified terms, can fluctuate from one period to another based on the economic environment and central bank interest rate decisions. Stock market returns are inherently volatile and subject to significant fluctuations based on market sentiment, economic data, and various other factors.

Because of these attributes, the passbook savings rate stands out as the most stable and least volatile option among the choices presented, making it the correct answer.