Explanation:

The 'C.P.U.' (Central Processing Unit) is considered the 'controlling part' of the computer because it directs and coordinates all operations, executes instructions, and manages the flow of data between various components. Option (c) 'C.P.U.' accurately identifies this central control function. Options (a) 'Printer' and (b) 'Keyboard' are peripheral input/output devices. Option (d) 'Hard disk' is a storage device. None of these perform the central control function of the CPU.