This is a 4:1 multiplexer. It has 4 input, one output and two select line.
Based on the value of select line, MUX will allow the input to pass. At a time only one input will be allowed to pass.
Select line. Input allowed to pass
0 0 I0
0 1 I1
1 0 I2
1 1 I3
The top two logic sources are select line and the remaining are the inputs.
