Explanation:

Hardware refers to the physical, tangible components of a computer system. These are the parts you can physically touch. Let's analyze each option: Option (a) 'An MP3 file' is a digital audio file format; it is a piece of data or software, not a physical component. Option (b) 'A keyboard' is a physical input device used to enter text and commands. Option (c) 'A monitor' is a physical output device that displays visual information. Option (d) 'A mouse' is a physical input device used to control a pointer on the screen. Since a keyboard, monitor, and mouse are all physical components, they are hardware. An MP3 file, being digital data, is not hardware. Thus, 'An MP3 file' is the correct answer as it is not an item of hardware.