The correct answer is D, AWS Organizations. AWS Organizations is a service that allows you to centrally manage multiple AWS accounts, including sharing resources like EC2 Reserved Instances across those accounts. This service provides a way to organize accounts into a hierarchy and apply policies to manage and control the use of AWS services and resources. In the context of the question, where departments need to share Reserved Instances, AWS Organizations is the appropriate tool to enable this sharing and centralize management of the AWS environment across departments.