To test the high availability of your Cloud SQL for PostgreSQL instance by performing a failover, you should use the gcloud sql instances promote-replica command on the replica instance. This command promotes the replica to be the new primary instance, effectively performing a failover. The correct usage is gcloud sql instances promote-replica <ReplicaInstanceName>, which corresponds to option D. Option A and B are incorrect because the failover command is not used in this context for Cloud SQL. Option C is incorrect because you should specify the replica instance name, not the primary instance name, when promoting a replica.