Your company is shutting down their data center and migrating several MySQL and PostgreSQL databases to Google Cloud. Your database operations team is severely constrained by ongoing production releas

An external read replica is a Cloud SQL concept where Cloud SQL itself replicates from an external primary; this is largely equivalent to option E but described less precisely, making E the more accurate and specific answer.

A standard read replica within Cloud SQL replicates data from a Cloud SQL primary instance, not from an on-premises database, so it cannot keep the Cloud SQL instance in sync with on-premises changes.

Database Migration Service automates the initial data migration and uses CDC to continuously replicate changes from on-premises MySQL and PostgreSQL databases to Cloud SQL, requiring minimal DBA effort and keeping the Cloud SQL instance in sync until application cutover. Option E (replication from an external server) is the underlying mechanism Cloud SQL uses to stay synchronized with an external source database during migration, enabling zero-downtime cutover by promoting the Cloud SQL instance when ready.

A cross-region read replica replicates data between Cloud SQL instances in different regions, not from on-premises databases to Cloud SQL.