The correct answer is WINDOWS.Among the options, only Windows is NOT an open-source operating system.Windows is a closed source operating system marketed by Microsoft.Microsoft first introduced the Windows operating system on 20 November 1985.Closed Operating Systems are built with numerous codes and complex programming and that is called source code. An Open-source Operating System is the Operating System in which source code is visible publically and editable.The operating system is a collection of programs that controls the overall operations of the computer.Operating system is a System software in computers.It works as an interface between user and computer hardware components.Examples of operating systems are:Linux.Unix.DOS.Windows.Ubuntu.Embedded operating system.OpenBSD.Mac OS.Confusion Points Windows is an operating system, but it is NOT an open-source operating system.