The correct answer is C because it aligns with Google Cloud's best practices for service account key management. For on-premises infrastructure accessing Google Cloud services, user-managed service account keys are required since GCP-managed keys cannot be downloaded and used externally. For GCE VMs running within Google Cloud, GCP-managed keys are preferred as they are automatically rotated, more secure, and require no manual key management. Option A is incorrect because provisioning service account keys for GCE VMs is unnecessary and less secure than using GCP-managed keys. Option B is incorrect as user accounts should not be used for service authentication due to security risks and lack of automated key rotation. Option D is overly complex and unnecessary, as custom authentication services are not required when standard service account mechanisms are available. The community discussion strongly supports C with high upvotes and references to official Google documentation.