Explanation:

The question asks about the fundamental composition of a byte, given that computer memory is measured in units like bytes, kilobytes, and megabytes. The smallest unit of information in computing is a bit, which is a binary digit (either $0$ or $1$). A byte is the most common fundamental unit of data storage and is universally defined as a collection of $8$ bits. Therefore, a byte is made up of eight binary digits. Option (a) 'Eight binary digit' correctly describes a byte. Option (b) 'Two binary digit' would be $2$ bits, which is not a byte. Options (c) 'Eight decimal digit' and (d) 'Two decimal digit' are incorrect because computers operate on binary digits, not decimal digits, at this fundamental level. Option (e) 'None of these' is incorrect as option (a) provides the correct composition.