Explanation:

The memory unit (specifically primary memory like RAM) is an integral part of the computer system, working in close conjunction with the Central Processing Unit (CPU). While physically separate, it is essential for the CPU's operation. Option (a) Control unit is part of the CPU. Options (c) and (d) are peripherals. Option (b) Central Processing Unit is the best fit, as memory is crucial for the CPU's functioning. Option (e) is incorrect.