The correct service for ongoing replication and real-time data synchronization is the Database Migration Service. When you need to move databases with minimal downtime and require continuous replication, this service is the optimal choice. It not only allows for the migration of data but also keeps the databases in sync during the migration process.

• The Snapshot feature within the Relational Database Service platform is primarily for backups, not for ongoing replication or real-time synchronization.
• The Schema Conversion Tool is mainly utilized for converting database schemas for migration purposes, but it does not handle real-time data replication.
• DataSync is more suitable for large-scale data transfer tasks and does not specialize in the specific requirements of ongoing database replication or synchronization.