Which of the following devices enables more than one signal to be sent out simultaneously...

Multiplexers are devices that enable enables more than one signal to be sent out simultaneously over one physical circuit.

Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 3: Telecommunications and Network Security (page 118).