Which cloud model is completely private to an organization and typically managed by one business unit while others make use of it?

The correct choice is the private cloud model, which is designed to be used exclusively by a single organization. This model provides a dedicated environment where the organization can have full control over its resources and data. A private cloud can offer a higher level of security and privacy compared to other cloud models, as it is not shared with other organizations.

In a private cloud, the management and infrastructure are typically handled by one business unit within the organization, allowing for tailored services and configurations that meet specific needs. This control is particularly important for organizations with strict compliance and regulatory requirements, as they can customize security measures according to their policies.

The private cloud model facilitates flexibility and scalability; organizations can allocate resources to different departments while still maintaining an overall private environment. This aspect makes it particularly appealing for larger enterprises that require both dedicated resources and shared services, as different business units can leverage the private cloud resources according to their needs without compromising security.