A. Modify the DB instance by using the RDS management console, and enable encryption. Apply the changes immediately.
B. Create a manual DB instance snapshot and then create an encrypted copy of that snapshot. Use this snapshot to create a new encrypted DB instance. Modify the application to connect to the new DB instance.
C. Create a new encrypted DB instance and use AWS Database Migration Service (AWS DMS) to migrate the existing database to the encrypted DB instance. Once the instances are in sync, modify the application to connect to the new DB instance.
D. Create an encrypted read replica. Once the read replica is in sync, promote it to primary. Modify the application to connect to the new primary instance.