Which solution will meet these requirements with the MOST operational efficiency?

Explanations:

While this option provides command session logging and uses Amazon CloudWatch for logging, it requires configuration on each EC2 instance individually. This adds operational overhead and does not leverage AWS Systems Manager, which offers a more efficient centralized management solution.

This option relies on a central bastion host for user access, which introduces a single point of failure and operational complexity. Additionally, managing session logs from a bastion host is less efficient than using AWS Systems Manager Session Manager, which can directly log sessions from multiple instances without requiring a bastion host.

This solution utilizes AWS Systems Manager Session Manager, which allows for secure access to EC2 instances without needing a bastion host. It automatically logs session data to Amazon CloudWatch Logs, and setting up metric filters and alarms in CloudWatch allows for effective monitoring of logs. This provides a highly efficient and centralized way to manage user sessions.

Although this option uses AWS Systems Manager for command execution and logs sessions, it requires configuration on each EC2 instance for command session logging. It also introduces additional complexity by using both Run Command and the CloudWatch agent, leading to more operational overhead compared to the streamlined approach of Session Manager in option C.