According to Google Cloud Architecture Framework and Cloud KMS documentation, when a customer requires direct control and auditability over the encryption keys used for data at rest, Customer-Managed Encryption Keys (CMEK) is the recommended path.

While Google Cloud encrypts all data at rest by default using Google-managed keys (Option B), Altostrat specifically requires control and auditability. By using CMEK through Cloud Key Management Service (Cloud KMS), Altostrat can manage the key rotation schedules, revoke access to keys immediately to "kill" access to the data, and view detailed logs in Cloud Audit Logs every time the key is used to encrypt or decrypt a media asset.

Option D is superior to Option C because client-side encryption with a self-managed HashiCorp Vault adds significant operational overhead and complexity, contradicting the business requirement to "Simplify infrastructure management." CMEK integrates natively with Cloud Storage, allowing for a seamless experience while meeting the strict security requirements for sensitive documentaries and interviews. Furthermore, the use of IAM roles and groups for fine-grained access control aligns with the principle of least privilege, ensuring that only authorized service accounts and users can access the sensitive buckets and the keys required to decrypt them.