We understand that the Boolean expression for this 4-to-1 Multiplexer with inputs A to D and data select lines a, b can be represented as:
Q = abA + abB + abC + abD
We 4 registers of 2 bits, the most recommended multiplexer is a 4-to-1:
In the representation below, at any one instant in time only ONE of the four analogue switches is closed, connecting only one of the input lines A to D to the single output at Q. As to which switch is closed depends upon the addressing input code on lines “a” and “b“.
Therefore, to select input B to the output at Q, the binary input address would need to be “a” = logic “1” and “b” = logic “0”.