A system administrator has launched Amazon EC2 instances and would like to create alarms that send notifications when CPU utilization thresholds are breached. Which AWS service would best meet their needs?

Amazon CloudWatch is the optimal service for this scenario since it is specifically designed to monitor and provide data about resources in AWS, including Amazon EC2 instances. It enables users to create alarms based on specific metrics, such as CPU utilization. By setting a threshold for CPU usage, CloudWatch can trigger alarms and notifications when these thresholds are exceeded, making it a perfect fit for the administrator's requirements of monitoring performance and receiving alerts.

While Amazon SNS (Simple Notification Service) is a valuable service for sending notifications, it does not independently monitor or evaluate metrics; rather, it works in conjunction with services like CloudWatch. EC2 Auto Scaling assists with managing the scaling of instances based on predefined performance metrics but is not used for generating alarms directly. Amazon CloudTrail is focused on logging and monitoring activity related to the AWS account and is not suited for monitoring performance metrics like CPU utilization. Thus, CloudWatch stands out as the most relevant service for the need described.