What actions can Amazon CloudWatch perform based on a defined metric that has been exceeded?

Amazon CloudWatch is a powerful monitoring service that allows users to collect and track metrics, collect log files, and set alarms. When a defined metric exceeds a specified threshold, Amazon CloudWatch can take automated actions based on the events that have been set up.

The correct answer highlights the actions that can be executed through CloudWatch in response to metric thresholds being crossed. It includes scaling an Auto Scaling group, which is crucial for maintaining application performance by adjusting the number of EC2 instances in response to load, ensuring high availability and efficiency. Additionally, sending messages to Amazon Simple Notification Service (SNS) allows you to notify relevant parties or trigger other actions based on the metrics. Managing EC2 instances could refer to stopping, starting, or restarting instances based on defined alarms.

The remaining options don't align with the capabilities of CloudWatch. For instance, executing a backup on Amazon S3 is not a predefined action that CloudWatch can directly execute upon metric thresholds being met. Similarly, shutting down all services and sending reports does not represent a specific and structured action provided by CloudWatch. Lastly, limiting access to AWS resources falls outside the scope of what CloudWatch is designed to do, as its primary function centers around monitoring and alerting rather than access control.