Explanation:

An iMac is a line of all-in-one personal computers designed and produced by Apple Inc. It is a complete computing system, integrating the display and computer components into a single unit. Therefore, it is best described as a 'Machine' ($ \text{d} $) or a complete computer system. Option (a) Processor is only one component of a computer (the CPU). Option (b) Modem is a device for modulating and demodulating signals for data transmission. Option (c) Network refers to a system of interconnected computers. An iMac is a fully functional computer machine, not just a part or a concept.