The correct answer is D, AWS X-Ray. AWS X-Ray is designed to help developers analyze and debug distributed applications, providing end-to-end visibility into how requests flow through an application's components. This includes tracing user requests as they move through Amazon API Gateway, AWS Lambda functions, and Amazon DynamoDB databases. X-Ray allows developers to identify performance bottlenecks, troubleshoot issues, and optimize the application's performance. The other options are not primarily designed for tracing user requests: AWS CloudTrail is for auditing API calls and events, Amazon CloudWatch is for monitoring AWS resources and applications, and Amazon Inspector is for security assessment to identify vulnerabilities.