JencoMart has decided to migrate user profile storage to Google Cloud Datastore and the application servers to Google Compute Engine (GCE). During the migration, the existing infrastructure will need access to Datastore to upload the data. What service account key-management strategy should you recommend?

The correct strategy is to provision service account keys for the on-premises infrastructure and use GCP-managed keys for the GCE VMs. On-premises systems cannot natively use GCP-managed service account keys, so they require a downloaded key file for authentication. GCE VMs, however, can leverage the built-in service account functionality, where Google manages the key rotation and distribution, enhancing security and reducing operational overhead. The other options are less secure or practical. Provisioning service account keys for both on-premises and GCE VMs increases the risk of key compromise on GCE. Authenticating on-premises with a user account is not suitable for programmatic access and lacks the granular permissions of a service account. Deploying a custom authentication service adds unnecessary complexity and maintenance burden compared to using native GCP authentication mechanisms.