Which of the following is NOT considered hardware?

The operating system is not classified as hardware because it is software that provides the necessary instructions for the computer's hardware components to communicate with each other. Essentially, it is the backbone of the computer's functionality, enabling users to run applications and manage files.

In contrast, the other items listed—mouse, keyboard, and monitor—are all physical devices that you can touch and see. They are essential components that facilitate user interaction with the computer system. The mouse and keyboard are input devices, allowing users to provide data and commands, while the monitor is an output device that displays information visually. Thus, understanding the distinction between hardware and software is crucial in recognizing their respective roles in computing systems.