1 to 4 De-multiplexer:
A 1 to 4 de-multiplexer (also known as a 1:4 demux) is a digital circuit that takes a single input and selects one of the four output lines based on the select input. It is the reverse of a multiplexer, which takes multiple inputs and selects one output line.

Functionality:
The 1 to 4 de-multiplexer has a single input line, four output lines, and two select input lines. The select input lines determine which output line to activate, based on the binary value of the select inputs. The number of select input lines determines the number of possible output lines that can be selected.

Number of Select Input Lines:
In a 1 to 4 de-multiplexer, two select input lines are required. Each select input line can take two different binary values, 0 or 1. Therefore, with two select input lines, we can have 2^2 = 4 possible combinations of select inputs, which corresponds to the four output lines of the de-multiplexer.

Explanation:
To understand why two select input lines are sufficient, let's consider an example. Assume we have a 1 to 4 de-multiplexer with output lines A, B, C, and D, and select input lines S1 and S0. The truth table for the de-multiplexer is as follows:

```
S1 | S0 | A | B | C | D
-----------------------
0 | 0 | I | 0 | 0 | 0
0 | 1 | 0 | I | 0 | 0
1 | 0 | 0 | 0 | I | 0
1 | 1 | 0 | 0 | 0 | I
```

In the truth table, I represents the input value that gets transmitted to the corresponding output line. For example, if S1 = 0 and S0 = 1, the input value will be transmitted to output line B.

As we can see from the truth table, each combination of select inputs corresponds to a unique output line. With two select input lines, we can cover all possible combinations of 0s and 1s, and hence, select any of the four output lines.

Therefore, the correct answer is option A) 2 select input lines.