Explanation:

The system unit is the main enclosure of a computer that houses its core components. The Central Processing Unit (CPU) is undeniably a fundamental and essential component located *inside* the system unit, mounted on the motherboard. Option (b) 'Monitor' is an output device that typically sits *outside* the system unit. Option (c) 'CD-ROM' refers to the optical disc media; if it implies a CD-ROM drive, then the drive is typically housed inside the system unit. However, the CPU is a more fundamental and universally present component of the system unit's core functionality. Option (d) 'Floppy disk' refers to a removable storage medium, not a component of the system unit itself. Given the options, the CPU (a) is the most accurate and universally recognized core part of the system unit. Option (e) 'None of these' is incorrect as (a) is correct.