Which one of the following is not a multi-user operating system?

A multi-user operating system allows multiple users to access a single computer system, either simultaneously or at different times, while maintaining individual user profiles and security. MS-DOS (Microsoft Disk Operating System) is a single-user, single-tasking operating system. It was developed for early personal computers and lacks the built-in capability to manage multiple concurrent users or run multiple programs simultaneously in a protected environment.

• UNIX: A classic example of a multi-user, multi-tasking system used in servers and workstations.
• Windows 2000 and Windows XP: Based on the Windows NT architecture, these systems support multiple user accounts, permissions, and fast user switching, making them multi-user operating systems.