Explanation:

The 'CPU' (Central Processing Unit) is widely referred to as the 'brain of the computer' because it is responsible for executing instructions, performing calculations, and managing the overall operations of the system. Option (a) 'CPU' accurately identifies this core component. Option (b) 'Monitor' is an output device. Option (c) 'Modem' is a communication device. Option (d) 'Software' is a set of instructions, not a physical component. Therefore, the CPU is the central processing and controlling unit.