The correct answer is D (Secret Manager) because it is specifically designed for securely storing and managing sensitive configuration data like API keys, passwords, and other secrets in Google Cloud. Secret Manager provides robust security features including encryption at rest, fine-grained access control through IAM, audit logging, and automatic secret rotation capabilities. It serves as a centralized, secure source of truth for secrets management. While options B (Compute Engine guest attributes) and C (Compute Engine custom metadata) can store configuration data, they are not designed for sensitive information and lack the security controls, encryption, and audit capabilities of Secret Manager. Option A (Cloud Key Management Service) is primarily for encryption key management rather than general secret storage. The community discussion strongly supports D with 100% consensus and high upvotes, emphasizing that Secret Manager is the purpose-built solution for this use case.