Amazon Bedrock ensures security and isolation of fine-tuned foundation models through encryption and account-level isolation. This means:

1. Encryption: All fine-tuned models are encrypted at rest and in transit using AWS encryption services.
2. Account Isolation: Each customer's fine-tuned models are isolated within their own AWS account, preventing cross-account access.
3. No Shared Registry: Unlike option A, models are not stored in a global shared registry.
4. No Manual Security Required: Unlike option C, security is not left to developers to manually configure with IAM.
5. No Automatic Deletion: Unlike option D, models are not automatically deleted after 30 days.

This approach aligns with AWS's shared responsibility model where AWS provides the security of the cloud infrastructure, and customers maintain control over their data and models within their accounts.