Which service should be used to provide the necessary information?

Explanations:

The AWS Personal Health Dashboard provides alerts and remediation guidance for AWS resources but does not provide insights into EC2 instance usage or Reserved Instance effectiveness.

AWS Cost Explorer helps analyze cost and usage data, including EC2 instances, and can provide insights into how Reserved Instances are being utilized and whether they are being effectively used.

AWS Service Catalog allows users to create and manage collections of AWS resources, but it does not provide detailed usage or cost data about EC2 instances or Reserved Instances.

AWS Application Discovery Service is used for collecting information about on-premises data centers to aid in migration to AWS, not for tracking EC2 usage or Reserved Instance effectiveness.