The most efficient method to track and bill costs for different departments sharing resources within a single AWS account is by using cost allocation tags. A tag is a key-value pair that can be attached to AWS resources. By creating a tag with a key like department and assigning the appropriate department name as the value to each resource, the company can activate these tags in the AWS Billing and Cost Management console. This allows for detailed cost tracking and reporting by department in tools like AWS Cost Explorer and in cost and usage reports, achieving the goal with the least operational overhead as it does not require any architectural changes.

B. Moving each department's resources to its own VPC is a significant architectural change that involves high operational overhead for migration and network management, which contradicts the requirement.
C. Migrating resources to separate AWS accounts for each department is a complex process with very high operational overhead, making it unsuitable for a "LEAST overhead" solution.
D. AWS Organizations is designed to manage multiple AWS accounts. Since all resources are in a single account, Organizations cannot be used to segregate costs within that account.

1. AWS Billing and Cost Management User Guide: "You can use tags to organize your resources

and cost allocation tags to track your AWS costs on a detailed level. After you activate cost allocation tags

AWS uses the cost allocation tags to organize your resource costs on your cost allocation report

so you can track your costs."

Source: AWS Billing and Cost Management User Guide

Section: "Using cost allocation tags".

2. AWS Well-Architected Framework - Cost Optimization Pillar (aws-well-architected-framework.pdf): "Implement a tagging strategy that allows you to identify and allocate costs to business units

projects

or cost centers. This enables you to perform detailed cost analysis and chargeback."

Source: AWS Well-Architected Framework

Cost Optimization Pillar

Section: "Cost-aware processes"

Page 6.

3. AWS Organizations User Guide: "AWS Organizations is an account management service that enables you to consolidate multiple AWS accounts into an organization that you create and centrally manage."

Source: AWS Organizations User Guide

Section: "What is AWS Organizations?".