Explanation:

A computer system is fundamentally defined by its ability to take input, process it, and produce output, often involving storage. The combination of input devices (like keyboards, mice), output devices (like monitors, printers), and processing devices (like the CPU) forms the core components of a functional computer system. Option (a) 'mobile device' is a specific type of computer system. Option (b) 'information processing cycle' describes the flow of operations, not the grouping of devices. Option (c) 'circuit board' is a hardware component, but not the complete system. Therefore, 'computer system' (d) is the most appropriate term for this grouping of devices.