A software team needs short-lived test environments for pull-request validation. Each environment is a single EC2 instance in an Auto Scaling group and must report results to a central server in the on-premises data center. A transit gateway with a VPN attachment to on-premises already exists. The process of creating and deleting environments must be fully automated and require minimal operational overhead. Which approach meets these requirements with the LEAST operational overhead?

The correct option minimizes operational overhead by centralizing networking. A single VPC with a pre-configured Transit Gateway attachment and routing simplifies network management for all test environments. CloudFormation then efficiently deploys and tears down individual EC2 instances and Auto Scaling groups within this existing network infrastructure, avoiding repeated network setup. Creating a new VPC for each test environment (option 1) or a new AWS account (option 3) introduces significant overhead due to the repeated provisioning and de-provisioning of network infrastructure and account management. Containerizing and using EKS (option 4) adds complexity and overhead with Kubernetes management, which is not necessary given the requirement for single EC2 instances.