Answer : 3. "C++"
Explanation :

Answer: C) C++ Explanation: C++ is not an Operating System.   An Operating System is a collection of system programs that manages all the other programs application programs in a computer as well as the allocation and use of hardware resources such as the CPU, Memory and the Hard Disk Drive. It acts as an interface between the hardware and the user level program. It controls and facilitates the overall operation of a computer.    Here UNIX : UNIX is a popular multi-user, multitasking operating system (OS) developed at Bell Labs.   Windows NT : Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993.   DOS : DOS is Disk Operating System. The term DOS can refer to any operating system.   C++ : C++ is a programming language and computing platform.   Hence, C++ is not an Operating System.