Which group contains only the root user?

The choice that contains only the root user is the wheel group. In Unix-based systems, the wheel group is a special user group that is typically assigned the highest level of access for performing administrative tasks. Members of this group, including the root user, have the ability to execute commands that affect the entire system, such as modifying system files or performing tasks that regular users cannot.

The wheel group is significant because it provides a means to control access to administrative privileges without giving full access to every user on the system. While the root user is inherently part of this group, in many configurations, it is restricted to just the root user to enhance security.

Other groups such as the admin, users, and staff groups typically contain multiple user accounts with varying permissions and roles for managing system resources and access, which differentiates them from the wheel group that is specifically tailored for superuser activities.