The correct approach to designing an IAM impersonation solution for a multi-tenant GKE cluster, following Google-recommended practices and the principle of least privilege (PoLP), involves using Workload Identity. Workload Identity allows you to assign specific Google service accounts to Kubernetes service accounts, which can then be mapped to individual workloads. This method eliminates the need for long-lived credentials by using short-lived tokens and ensures that each workload has only the permissions it needs. Option C correctly outlines this process: creating a Google service account, creating a Kubernetes service account in a Workload Identity-enabled cluster, linking them with the appropriate IAM role and annotation, and then mapping the Kubernetes service account to the workload. This approach is secure, scalable, and adheres to Google's best practices. Options A and B do not utilize Workload Identity and thus do not fully address the customer's concerns about long-lived credentials. Option D involves creating and managing service account keys, which introduces the risk of long-lived credentials and does not follow the recommended practices.