AWS Trusted Advisor is correct because it provides automated best practice recommendations and includes a Low Utilization Amazon EC2 Instances check that reviews recent activity and flags instances with consistently low CPU and minimal network traffic to help reduce EC2 costs.

The AWS Trusted Advisor low utilization check analyzes CPU and network usage over multiple days and highlights candidates for stop, termination, or downsizing so you can quickly act on cost savings. It surfaces concise recommendations across accounts and removes the need to manually scan raw metrics when the question asks for an automated tool that pinpoints underutilized instances.

AWS Cost Explorer is focused on spending analysis and cost trends and it does not automatically analyze instance performance to identify underused EC2 instances.

Amazon CloudWatch provides detailed metrics and alarms for CPU and network but it requires you to interpret those metrics and it does not itself provide a built in underutilization recommendation for EC2.

AWS Budgets monitors and alerts on budget thresholds and forecasts and it is not designed to give resource efficiency or right sizing insights.

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