An application runs on EC2 instances in an Auto Scaling group and experiences variable daily load. The company needs detailed EC2 instance metrics to right-size instances and must also monitor custom application metrics. Which solution satisfies both needs?

The Amazon CloudWatch agent is the correct solution because it is designed to collect both detailed EC2 instance metrics (including memory and disk utilization, which are not available by default CloudWatch metrics) and custom application metrics from within the instances. It can be configured to send these metrics to CloudWatch, enabling comprehensive monitoring and analysis for right-sizing and performance optimization. AWS X-Ray is for distributed tracing and analyzing application performance, not for collecting EC2 instance or custom application metrics directly. The AWS SDK can publish custom metrics, but it's not the primary tool for comprehensive EC2 instance metric collection and would require significant application modification for basic instance metrics. AWS CloudTrail records API calls and events, not performance metrics from within instances.