The correct answer is D. Notepad.
An operating system is a software that manages computer hardware and software resources and provides a platform for running application programs. It is the most important program that runs on a computer. Without an operating system, a computer would be useless.
Windows 2000, Linux, and Mac OS are all operating systems. Windows 2000 is a discontinued operating system developed by Microsoft. Linux is a Unix-like operating system that is developed and maintained by a community of developers. Mac OS is a Unix-based operating system developed and marketed by Apple Inc.
Notepad is a text editor that is included with Microsoft Windows. It is a simple program that allows
users to create and edit text files. Notepad is not an operating system because it does not manage computer hardware and software resources or provide a platform for running application programs.