Option B is correct because migrating the databases to Amazon RDS with encryption at rest configured directly addresses the requirements of enhancing security and reducing operational overhead. Amazon RDS provides automated backup, software patching, and database management, which reduces operational overhead, while encryption at rest ensures that the sensitive data is secure. Option A involves managing EC2 instances and associated infrastructure, which increases operational overhead. Options C and D do not directly address the need for transactional database management and related security at rest.