AWS Organizations is a service that helps you centrally manage and govern your AWS environment. You can use AWS Organizations to create multiple accounts for different business units, and group them into organizational units (OUs) that reflect your organizational structure1. By doing so, you can separate and track costs for each business unit using the account ID as a cost allocation tag2. You can also use AWSOrganizations to apply policies and controls to your accounts, such as service control policies (SCPs) and tag policies1.

The other options are not suitable for meeting the requirements with the least operational overhead. Using a spreadsheet or a DynamoDB table to control and record costs for each business unit would require manual data entry and maintenance, which is prone to errors and inconsistencies. Using the AWS Billing console to assign owners to resources and track costs would also require manual tagging of each resource, which is time-consuming and inefficient.

1: What Is AWS Organizations? - AWS Organizations

2: Cost Tagging and Reporting with AWS Organizations | AWS Cloud Financial Management