How will I justify Windows is a multiuser operating system?

That would depend on how you define multiuser:

  • in the traditional timesharing sense and as Multics, Unix and VMS was designed to be multiuser, where it means many people may login to one computer at the same time via terminals connected to that computer, Windows is clearly NOT multiuser.
  • As a system where many user accounts can exist on one computer and any of these people can login ONE AT A TIME and can access servers through a network that other users can access at the same time, Windows is multiuser.