The correct answer is C. This solution involves creating credentials for the application user on the RDS MySQL database and securely storing these credentials in AWS Secrets Manager. The application will be configured to retrieve the credentials from Secrets Manager, and a credentials rotation schedule will be set up using Secrets Manager. This approach provides a managed solution for credential storage and rotation with minimal programming effort, ensuring enhanced security. Option A, while secure, involves more effort in key management and integration. Option B requires additional implementation of a Lambda function, which increases complexity. Option D involves AWS Systems Manager Parameter Store, but AWS Secrets Manager is a more suitable service for managing database credentials, including rotation.