What is one issue associated with bond indexes?

The issue associated with bond indexes that is highlighted in the correct answer relates to how the risk characteristics of the index can often mirror the preferences of borrowers rather than the diversifying nature of the bond market. This implies that the index may not accurately represent the full spectrum of risks inherent in the broader bond market, which can include variations in credit risk, interest rate risk, and other market dynamics. As a result, bond indexes might not effectively capture the movements and behaviors of different types of bonds, limiting their utility as benchmarks for diverse fixed-income investing strategies.

When evaluating the other options, it's important to note that while bond indexes can indeed change composition over time—reflecting market conditions and changes to the set of included bonds—aspects like consistent maturity dates do not typically apply, as bond indexes generally contain bonds with varied maturities to track market segments effectively. Additionally, while some indexes may maintain relatively stable compositions, this does not universally characterize all bond indexes, making the other choices less relevant in this context.