The correct answer is C, AWS CloudTrail. AWS CloudTrail is a service that provides a history of actions taken in an AWS account, including API calls made to AWS services. It allows you to monitor and retain account activity logs for review. In this scenario, the system administrator can use CloudTrail to identify who made the API call to terminate the EC2 instance by examining the logs, which will show the user or service that made the call, the action performed, and the source IP address. This service is specifically designed for auditing and tracking resource changes within AWS, making it the most suitable choice for this purpose.