Explanation:

This question asks for a fundamental definition in computer science regarding the relationship between bits and bytes. \nOption (a) 8: This is the correct answer. By standard definition in computer architecture and digital information, one byte is composed of $8$ bits. This grouping of $8$ bits allows for $2^8 = 256$ distinct values or characters to be represented, which is a convenient number for encoding alphanumeric characters, symbols, and small integers. This standard was established early in computing history and remains widely adopted. \nOption (b) 16: This is incorrect. $16$ bits constitute two bytes, often referred to as a 'word' or 'short integer' in some architectures, but not a single byte. \nOption (c) 64: This is incorrect. $64$ bits represent eight bytes ($64 \text{ bits} \div 8 \text{ bits/byte} = 8 \text{ bytes}$). This is a common word size for modern $64$-bit processors, but it is not the definition of a single byte. \nOption (d) 256: This is incorrect. $256$ is the number of distinct values that can be represented by one byte (i.e., $2^8$), not the number of bits in a byte. \nOption (e) 512: This is incorrect. $512$ bits would be $64$ bytes ($512 \text{ bits} \div 8 \text{ bits/byte} = 64 \text{ bytes}$). The mathematical relationship is $1 \text{ Byte} = 8 \text{ bits}$.