Which of the following is not an operating system?

An operating system (OS) is system software that manages a computer's hardware and provides services to other programs: it controls memory, processes, files, and input/output devices, and gives the user an interface to that hardware. A compiler is different in kind: it is a programming tool that translates source code written in a high-level language into machine code (or another lower-level form) so a processor can run it; it does not manage hardware itself.

Among the four options, MS-DOS, the Macintosh operating system (MAC), and Linux each manage a computer's hardware, memory, and processes and provide the base platform on which other software runs, so all three are operating systems. A compiler, by contrast, is an application-level tool that runs on top of an operating system to convert code; it manages no hardware. So COMPILER is the one that is not an operating system.

• MS-DOS — a disk operating system for IBM-compatible PCs; manages files, memory, and program execution via command-line instructions.

• MAC (the Macintosh operating system) — Apple's operating system for Mac computers; manages hardware and provides the graphical desktop environment.

• COMPILER — a program that translates source code into machine code; a software-development tool, not an operating system.

• LINUX — an open-source, Unix-like operating system kernel; manages hardware resources for servers, desktops, and countless distributions.

Therefore, the correct answer is COMPILER: it is a language translator, not an operating system.