Third-party audit reports (D) provide the most reliable evaluation of a cloud provider's security posture because they offer independent assurance of control design and effectiveness. Peer reviews (A) and attestations (B) are less objective, while penetration test reports (C) focus narrowly on technical vulnerabilities. CISM emphasizes independent assurance mechanisms when assessing third-party security, particularly for cloud services. References: ISACA CISM Review Manual (Governance-third-party assurance and cloud risk); CISM Exam Content Outline (Domain 2).