B. SSM OpsCenter is primarily used for managing and resolving operational issues. It is not designed to securely store and manage credentials like AWS Secrets Manager.
C. Storing credentials in an S3 bucket may provide some level of security, but it lacks the additional features and security controls offered by AWS Secrets Manager.
D. While using KMS for encryption is a good practice, managing credentials directly on the web server file system can introduce complexities and potential security risks. It can be challenging to securely manage and rotate credentials across multiple web servers, especially when considering scalability and automation.
In summary, option A is the recommended solution as it leverages AWS Secrets Manager, which is purpose-built for securely storing and managing secrets, and provides the necessary IAM permissions to allow the web servers to access the credentials securely.