Explanation:

The question lists Unix, MS DOS, and Linux and asks for the category they belong to. Unix is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix. MS-DOS (Microsoft Disk Operating System) is a single-user, single-tasking operating system that was dominant on IBM PC compatible computers. Linux is a family of open-source Unix-like operating systems based on the Linux kernel. All three, Unix, MS DOS, and Linux, are fundamental software that manage computer hardware and software resources, providing common services for computer programs. This definition perfectly aligns with the concept of an operating system. Therefore, option (c) Operating Systems is the correct classification. Options (a) Computers, (b) Brands of printers, and (d) Printers are clearly incorrect, as these are hardware or hardware-related categories, not software types.