Which of the following is not an operating system?

Introduction / Context:
Operating systems and programming languages play very different roles in computer systems. Operating systems manage hardware and provide services to programs, while languages are used to write those programs. This question tests your ability to distinguish between names of operating systems and the name of a widely used programming language.

Given Data / Assumptions:

• Some of the listed items are well known operating systems.
• One of the items is a general purpose programming language.
• The task is to identify the one that is not an operating system.
• No numerical analysis is required.

Concept / Approach:
UNIX is a family of multitasking, multiuser operating systems originally developed at Bell Labs. Windows NT is a line of operating systems developed by Microsoft for servers and desktops. DOS, or disk operating system, refers to several related operating systems, including MS DOS. C plus plus is a general purpose programming language used to write software, including operating systems and applications, but it is not itself an operating system. Therefore, among the choices, C plus plus is the one that does not belong to the category of operating systems.

Step-by-Step Solution:
Step 1: Identify UNIX as an operating system widely used on servers and workstations. Step 2: Recognise Windows NT as a Microsoft operating system used on desktops and servers. Step 3: Recall that DOS stands for disk operating system and is indeed an operating system family. Step 4: Note that C plus plus is taught as a programming language used to implement software. Step 5: Conclude that C plus plus is the only option that is not an operating system.

Verification / Alternative check:
Documentation and historical references clearly describe UNIX, Windows NT and DOS as operating systems with kernels, shells and system utilities. In contrast, C plus plus appears in programming language references and standard library specifications, not in lists of operating systems. This provides clear evidence that C plus plus is the correct choice for the item that is not an operating system.

Why Other Options Are Wrong:
Option A, UNIX, is an operating system and therefore does not satisfy the requirement of being something else. Option B, Windows NT, is also an operating system, and option D, DOS, is an operating system family as well. Only option C is a programming language and not an operating system, so the others are incorrect answers to the question posed.

Common Pitfalls:
A common mistake is to focus only on familiarity; students may recognise C plus plus as famous and forget that the question is about operating systems. Another pitfall is to confuse the language in which an operating system is written with the operating system itself; for example, parts of an operating system might be implemented in C or C plus plus, but the language remains distinct from the operating system product.

Final Answer:
C plus plus is not an operating system; it is a programming language.