The correct answer is D, Amazon CloudWatch. Amazon CloudWatch is a monitoring service for AWS resources and the applications you run on AWS. It can be used to create alarms based on metrics such as CPU utilization. When the CPU utilization of an EC2 instance exceeds a predefined threshold, an alarm can be triggered to initiate an Amazon EC2 Auto Scaling action. This action can add or remove instances to maintain the desired performance level and manage costs. Amazon SNS (B) is a notification service that can be used in conjunction with CloudWatch alarms to send notifications, but it does not directly trigger Auto Scaling actions. Amazon SQS (A) is a managed message queue service and AWS Systems Manager (C) is used for patching and managing operational tasks, neither of which are used to trigger Auto Scaling actions based on CPU utilization.