An independent assessment against a relevant standard (B) provides the greatest assurance because it offers objective, third-party validation that security controls and development practices meet defined criteria. Examples include assessments against secure development standards or recognized control frameworks. Training (A), certifications (C), and policy reviews (D) are useful inputs, but they are indirect and do not confirm consistent, effective implementation. CISM emphasizes independent assurance for third-party risk management, especially when outsourcing high-risk activities such as custom software development. Independent assessments reduce reliance on self-attestation and help validate governance, SDLC controls, and security testing practices.