Explanation:

Computers, at their most fundamental level, operate using electrical signals that are either 'on' or 'off'. This two-state nature directly corresponds to the binary number system, which uses only two digits: $0$ and $1$. Option (a) 'binary' is the correct answer because all data, instructions, and operations within a computer's central processing unit (CPU) are ultimately represented and processed using binary code. Option (b) 'octal' (base-$8$), (c) 'decimal' (base-$10$), and (d) 'hexadecimal' (base-$16$) are number systems that humans often use to represent binary data in a more compact and readable form, but the computer itself does not natively work in these systems for its internal operations. Option (e) 'none of these' is incorrect as option (a) is correct.