Which combination of steps should the security team take?

Explanations:

The Systems Manager Agent (SSM Agent) must be installed and running on all EC2 instances to enable communication with AWS Systems Manager. Without the agent, the instances cannot be managed or accessed via Session Manager.

The IAM role attached to the EC2 instances must have permissions that allow access to Systems Manager. This includes policies that grant necessary permissions for the instances to use Session Manager and other Systems Manager features.

While creating a Service Control Policy (SCP) to prevent the creation of SSH key pairs might seem like a measure to enforce security, it does not directly impact the ability to use AWS Systems Manager for management and access. SCPs are used to manage permissions for AWS accounts, but they do not apply to the actions taken within a single account.

A NAT gateway is not needed because the EC2 instances are not connected to the internet, and Systems Manager can communicate with the AWS services through VPC endpoints. Therefore, setting up a NAT gateway is unnecessary for this use case.

Ensuring proper VPC endpoints for Systems Manager and Amazon EC2 is essential because it allows the EC2 instances to communicate with AWS Systems Manager without requiring internet access. VPC endpoints enable private connections between the VPC and the services.

A transit gateway is not required for the setup described, as it primarily facilitates connections between multiple VPCs and on-premises networks. In this scenario, the focus is on using Systems Manager without internet access, which can be achieved without a transit gateway.