Which user has the user ID (UID) of 0?

In Unix and Linux systems, the user ID (UID) of 0 is conventionally assigned to the root user. This user has the highest level of access and privileges on the system, allowing complete control over all aspects of the operating system. The root account can perform administrative tasks such as installing software, modifying system configurations, and managing user accounts, which are restricted for normal users.

While the other choices may refer to various user roles within a system, they do not possess the unique characteristics of the root user. The admin user, for instance, may have elevated privileges but does not necessarily have a UID of 0. Similarly, system users typically are assigned UIDs greater than 0 for specific functions, while the guest user is a temporary account often created for limited access. Therefore, the root user is correctly identified as having the UID of 0, signifying its role as the superuser in the system.