The correct answer is A, AWS CloudTrail. AWS CloudTrail is a service that provides a way to log, continuously monitor, and retain account activity related to actions taken in AWS. It records API calls made by or on behalf of the AWS account and delivers those log files to an Amazon S3 bucket for storage. This service is essential for maintaining visibility and control over your AWS environment, as it helps with security analysis, resource change tracking, and compliance auditing. The other options, while important services within AWS, do not specifically focus on auditing API calls: B) AWS Trusted Advisor provides guidance on cost optimization, performance, and security; C) Amazon Inspector is an automated security assessment service; and D) AWS X-Ray is used for analyzing and debugging distributed applications.