A company with multiple business units wants centralized governance: automate account creation, enforce service control policies, and simplify billing. Which AWS service should they use?

AWS Organizations is the correct choice because it allows you to centrally manage and govern your environment as you grow and scale your AWS resources. It enables you to automate account creation, apply Service Control Policies (SCPs) to enforce permissions across accounts, and consolidate billing for all accounts into a single bill, addressing all the requirements. Cost Explorer is used for visualizing and managing AWS costs and usage, but doesn't offer governance or account management. AWS Budgets helps you set custom budgets and receive alerts when costs exceed thresholds, but it's not for organizational governance. AWS Trusted Advisor provides recommendations to optimize costs, security, performance, and fault tolerance, but it doesn't manage accounts or enforce policies across an organization.