Which of the following must be implemented by a cloud provider to ensure that different entities can authenticate and share basic user accounting information?

Scalability refers to the ability to handle increasing workloads or users, which is a desirable characteristic of cloud services but not directly about authentication and sharing user information.

Federation is a specific identity management mechanism that enables authentication and information sharing-between distinct identity domains, but virtualization provides the underlying infrastructure that hosts these distinct domains.

Virtualization is a fundamental technology in cloud computing that enables providers to create isolated virtual environments for different entities, such as tenants or customers. This isolation is crucial for each entity to establish and manage its own distinct authentication systems and user accounting information securely within the cloud provider's shared physical infrastructure.

Self-service allows users to provision and manage cloud resources independently, which is a cloud characteristic but unrelated to cross-entity authentication or sharing user accounting information.