Explanation:

In computer science, a byte is a fundamental unit of digital information, universally defined as a group of eight bits. This is a core concept in data storage and processing. Option (b) Byte is correct. Option (a) Bit is the smallest unit, a single $0$ or $1$. Option (c) Number is a general mathematical concept. Option (d) Kilobyte is $1024$ bytes, which is $1024 \times 8 = 8192$ bits, far more than eight bits. Therefore, 'Byte' precisely describes a series of eight bits, serving as a convenient grouping for data.