Explanation:

Character encoding standards, such as ASCII (American Standard Code for Information Interchange) or UTF-8, define how characters (letters, numbers, symbols) are represented in binary form. In many common encodings, a single character is represented by a unique combination of $8$ bits, which collectively form one byte. Option (a) 'eight bytes' would be $8 \times 8 = 64$ bits, which is typically too large for a single character in standard encodings. Option (b) 'eight kilobytes' is an extremely large amount of data, far beyond what's needed for one character. Option (c) 'eight characters' is incorrect; a character is made of bits, not other characters. Option (d) 'eight bits' is the correct answer, as $8$ bits form a byte, which is the standard size for representing a single character in many computer systems. Option (e) 'none of these' is incorrect as option (d) is accurate.