Explanation:

The CPU (Central Processing Unit) is primarily composed of the 'Control unit' (a) and the 'ALU' (Arithmetic Logic Unit) (c), along with registers. Both the Control Unit and the ALU are indispensable main components. Since options (a) and (c) are both main components, and (d) 'All' is provided, it implies that the question is asking to acknowledge both as main components. 'Memory' (b) is closely associated but typically considered external to the CPU's core functional units (though cache memory is internal). Thus, 'All' is the most appropriate choice to include the key functional components listed.