First Principles Thinking: decision rule
C is correct. The key criterion is the source of investor protection. The CFA Curriculum links lower credit risk and lower yields in covered bonds to dual recourse, strict eligibility criteria, dynamic cover pools, and redemption regimes. Those protections make the combined-protection choice the most accurate.
Why option A is incorrect. Covered bonds usually consist of one bond class per cover pool rather than a multi-tranche default-exposure structure.
Why option B is incorrect. That description belongs to true securitization, not covered bonds, and it does not preserve sponsor exposure in the same way.