To understand why the correct answer is option 'B', let's first discuss what a full adder is and how it can be used to construct an m-bit parallel adder.

A full adder is a digital circuit that takes in three inputs, A, B, and carry-in (Cin), and produces two outputs, sum (S) and carry-out (Cout). It can be represented using a truth table as follows:

| A | B | Cin | S | Cout |
|---|---|-----|---|------|
| 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 | 0 |
| 0 | 1 | 0 | 1 | 0 |
| 0 | 1 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 | 0 |
| 1 | 0 | 1 | 0 | 1 |
| 1 | 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 | 1 |

To construct an m-bit parallel adder, we need to add two m-bit binary numbers together. Each bit of the two numbers can be added using a full adder. The carry-out of each full adder is then propagated to the carry-in of the next full adder.

Let's consider the case of a 4-bit parallel adder, where m = 4. We can represent the inputs A and B as A[3:0] and B[3:0] respectively, where A[3] is the most significant bit and A[0] is the least significant bit.

To add A[3:0] and B[3:0] together, we need four full adders. The inputs to the full adders are as follows:

- A[0] and B[0] are connected to the inputs A and B of the first full adder.
- A[1] and B[1] are connected to the inputs A and B of the second full adder, and the carry-out (Cout) of the first full adder is connected to the carry-in (Cin) of the second full adder.
- Similarly, A[2] and B[2] are connected to the inputs A and B of the third full adder, and the carry-out (Cout) of the second full adder is connected to the carry-in (Cin) of the third full adder.
- Finally, A[3] and B[3] are connected to the inputs A and B of the fourth full adder, and the carry-out (Cout) of the third full adder is connected to the carry-in (Cin) of the fourth full adder.

In general, for an m-bit parallel adder, we need m full adders. Each full adder is responsible for adding one bit of the two numbers being added.

Hence, the correct answer is option 'B', m full adders are needed to construct an m-bit parallel adder.