Explanation:

This question asks for the most fundamental and smallest unit of data in computer memory. \nOption (b) Bit: This is the correct answer. A bit (short for 'binary digit') is the most fundamental and smallest unit of data in a computer. It can hold only one of two values: $0$ or $1$. All other units of computer memory (like bytes, kilobytes, megabytes, etc.) are composed of multiple bits. \nOption (a) Byte: A byte is a common unit of computer memory, but it is composed of $8$ bits. Therefore, it is not the smallest unit. \nOption (c) Megabyte: A Megabyte is a much larger unit of memory, consisting of $2^{20}$ bytes or $8 \times 2^{20}$ bits. It is far from the smallest unit. \nOption (d) These all: This is incorrect because only 'Bit' is the smallest unit among the options provided. Byte and Megabyte are larger units built upon bits. \nOption (e) None of these: This is incorrect as 'Bit' is indeed the smallest unit of computer memory. The hierarchy of units is: $\text{Bit} \rightarrow \text{Byte} \rightarrow \text{Kilobyte} \rightarrow \text{Megabyte} \rightarrow \text{Gigabyte} \rightarrow \text{Terabyte} \dots$