Multiplexer- The multiplexer is a combination logic circuit designed to switch one of several input lines to a single common output line. 

Multiplexers operate like very fast acting multiple position rotary switches connecting or controlling multiple input lines called channels one at a time to output. 

The Boolean expression for tis 4x1 multiplexer is , Q = abA + abB + abC + abD

Truth Table For This 4x1 Multiplexer