Explanation:

The 'ALU' (Arithmetic Logic Unit) is the specific component within the CPU that is dedicated to performing all 'calculating' (arithmetic operations like addition, subtraction) and 'comparing' (logical operations like greater than, less than, equality). Option (a) 'ALU' precisely describes this function. Option (b) 'Control unit' manages and coordinates operations but doesn't perform calculations itself. Option (c) 'Disk unit' is for data storage. Option (d) 'Modem' is for network communication. Therefore, the ALU is the correct answer.