The Multiplexer:-

Multiplexing is the generic term used to describe the operation of sending one or more analogue or digital signals over a common transmission line at different times or speeds and as such, the device we use to do just that is called a Multiplexer.

Basic Multiplexing Switch:-

Truth table:-

4 Channel Multiplexer using Logic Gates:-

This is full adder using half adder. Here even truth table is given for your conveniency.