Which highly scalable AWS service should a company use to monitor the performance and operational metrics of its machine learning systems?

Amazon CloudWatch is the correct choice because it is a monitoring and observability service built for AWS resources and applications, including machine learning systems. It collects and tracks metrics, collects and monitors log files, and sets alarms, making it ideal for performance and operational monitoring. AWS CloudTrail records API calls and related events, focusing on governance, compliance, and auditing, not performance metrics. AWS Trusted Advisor provides recommendations for cost optimization, security, fault tolerance, performance, and service limits, but it's not a real-time monitoring service for operational metrics. AWS Config enables you to assess, audit, and evaluate the configurations of your AWS resources, which is different from monitoring operational performance.