Explanation:

In computer terminology, a 'nibble' (sometimes spelled 'nybble') is a specific unit of data. We know that a 'byte' is composed of $8$ bits. A nibble is defined as a group of $4$ bits. Since $4$ bits is exactly half of $8$ bits, a nibble is equivalent to half a byte. Options (b) A terabyte, (c) A gigabyte, and (d) A kilobyte are all very large units of data storage, representing $10^{12}$, $10^9$, and $10^3$ bytes respectively (approximately), and are therefore incorrect. The term 'nibble' specifically refers to a $4$-bit unit, making (a) Half a byte the correct definition.