Explanation:

The three main functional parts of a CPU (processor) are the 'ALU' (Arithmetic Logic Unit), the 'Control unit' (CU), and 'Registers'. Option (a) 'ALU, Control unit and Registers' correctly identifies these core components. Registers are small, high-speed storage locations within the CPU that hold data and instructions during processing. Options involving 'RAM', 'Cache', 'ROM', or 'CD-ROM' refer to types of memory or storage that are external to the core functional units of the processor itself, even though cache is often integrated closely with the CPU.