First Principles Thinking: index composition mechanism
A is correct. The CFA Curriculum states that bond index constituents are usually weighted by market value of debt outstanding. Because bonds are issued by many issuer types, broad bond indexes reflect changes in bond market composition, including public versus private issuer debt, longer maturities, and deterioration in credit quality. The reason identifies the weighting mechanism that causes the index to reflect those market composition changes.
B is incorrect because the reason is not detached from the assertion; it explains why broad bond indexes move with debt market composition.
D is incorrect because the assertion matches the stated behavior of broad bond indexes.