The correct answer is C. Unix.
Unix is a multitasking, multiuser computer operating system that exists in many variants. The original Unix was developed at AT&T’s Bell Labs research center by Ken Thompson, Dennis Ritchie, and others. Unix was first released in 1971, and it has been widely adopted in academia, business, and government.
Linux is a Unix-like operating system that was developed by Linus Torvalds in the early 1990s. Linux is free and open-source software, and it is available for a wide range of hardware platforms. Linux is often used as a server operating system, but it can also be used on personal computers.
DOS is a family of operating systems that were developed by Microsoft in the 1980s. DOS was the first widely used operating system for personal computers. DOS was a single-user, single-tasking operating system.
GNU is a project that was started in the early 1980s by Richard Stallman. The goal of the GNU project is to create a free software operating system. The GNU operating system is not complete, but it includes many important components, such as the GNU Compiler Collection (GCC) and the GNU Emacs text editor.
E is not the correct answer because it is not an operating system.