Which AWS service should a solutions architect use to keep a fleet of EC2 instances healthy by automatically replacing any impaired instances?

AWS Auto Scaling is the correct choice because it allows you to automatically adjust the number of EC2 instances in your application based on demand and defined health checks. If an instance becomes impaired, Auto Scaling can terminate it and launch a new, healthy instance to maintain desired capacity and availability. Amazon ECS is a container orchestration service, not directly for managing EC2 instance health. Amazon GuardDuty is a threat detection service, and AWS Shield is a DDoS protection service; neither addresses the automatic replacement of unhealthy EC2 instances.