A company uses AWS Organizations and enforces a tagging policy that adds department tags to resources when they are created. The accounting team must determine Amazon EC2 costs by department across all AWS accounts in the organization. The accounting team has Cost Explorer access for all accounts and needs to view all reports from Cost Explorer. Which solution provides this MOST operationally efficiently?

The correct solution is to activate the user-defined cost allocation tag named 'department' from the Organizations management account billing console. Cost allocation tags, whether user-defined or AWS-defined, must be activated in the billing console before they can be used for cost analysis in Cost Explorer. Activating it in the management account ensures that the tag is available across all linked accounts in the organization, which is crucial for consolidated billing and reporting. Creating a single report grouped by this tag and filtered for EC2 provides the most operationally efficient way to analyze costs by department. Incorrect options: Activating an AWS-defined tag named 'department' is incorrect because 'department' is a custom tag created by the company, making it user-defined, not AWS-defined. AWS-defined tags are prefixed with aws:. Activating the tag from a member account billing console is incorrect because cost allocation tag activation for consolidated billing should be done from the management account to apply across all linked accounts. Activating it in a member account would only apply to that specific account.