Which of the following is not hardware of a computer system?

Answer: The operating system is not hardware.

Explanation: Hardware refers to the physical components of a computer that you can touch. Software refers to programs and operating systems that run on the hardware.

• Keyboard — a physical input device; it is hardware.

• Printer — a physical output device; it is hardware.

• Operating System — system software that controls and manages hardware and provides services to applications; it is not hardware.

• Mouse — a physical pointing/input device; it is hardware.

Therefore, the correct answer is the operating system because it is software, not a physical component.