Which AWS service should be used to determine what action made this EC2 instance inaccessible?

Explanations:

Amazon CloudWatch Logs primarily monitors and logs operational metrics and logs from AWS services and applications. While it can provide insight into performance and operational issues, it does not specifically track changes made to resources like EC2 instances.

AWS Security Hub aggregates and analyzes security alerts and findings from various AWS services. It focuses on security best practices and does not provide detailed tracking of actions taken on specific AWS resources like EC2 instances.

Amazon Inspector is a security assessment service designed to help improve the security and compliance of applications deployed on AWS. It does not track actions or changes made to EC2 instances, so it would not provide information on why an instance is inaccessible.

AWS CloudTrail records AWS API calls for accounts, providing event history of actions taken on AWS resources. It allows users to track changes and actions that could have caused an EC2 instance to become inaccessible, making it the best choice for determining the cause of the issue.