Your company wants to start using Google Cloud resources but wants to retain their on-premises Active Directory domain controller for identity management. What should you do?

The correct approach is to use Google Cloud Directory Sync (GCDS) to synchronize your on-premises Active Directory users and groups with Google Cloud identities. This creates corresponding Google identities without migrating the actual domain controller. Then, configure Security Assertion Markup Language (SAML) Single Sign-On (SSO) between Google Cloud and your on-premises Active Directory Federation Services (AD FS) or another SAML identity provider. This allows users to authenticate against their existing Active Directory credentials when accessing Google Cloud resources. Using the Admin Directory API directly for authentication against an on-premises AD is not a standard or secure integration method. Cloud Identity-Aware Proxy (IAP) is for securing access to applications, not for synchronizing identities or acting as a primary identity provider itself. Replicating an on-premises AD domain controller to Compute Engine is a valid strategy for extending AD to the cloud, but it doesn't directly address the requirement of retaining the on-premises domain controller for identity management while integrating with Google Cloud. The core need is synchronization and federated authentication, which GCDS and SAML SSO provide.