The correct answer is AWS CloudTrail, as it provides event history and a detailed log of who did what on your account, including actions taken through various interfaces. CloudTrail is specifically designed for auditing and reviewing historical data related to API calls. Amazon CloudWatch focuses on monitoring and logging operational metrics rather than API calls. Amazon GuardDuty is used for threat detection and continuous monitoring but does not have the same auditing capabilities as CloudTrail. AWS Config is more for tracking resource changes and configurations over time, and while it can generate logs, it's not as comprehensive for auditing all user activities and API usage as CloudTrail.