Which of the following devices enables more than one signal to be sent out simultaneously over one physical
circuit?

A.
Router

B.
Multiplexer

C.
Channel service unit/Data service unit (CSU/DSU)

D.
Wan switch

Explanation:
An electronic multiplexer makes it possible for several signals to share one device or resource. A multiplexer
(or mux) is a device that selects one of several analog or digital input signals and forwards the selected input
into a single line.
Incorrect Answers:
A: A router forwards data packets. A router does not handle signals.
C: A CSU/DSU is a digital-interface device used to connect a data terminal equipment (DTE), such as a router,
to a digital circuit, such as a Digital Signal 1 (T1) line.
D: A switch forwards traffic at the data link layer of the OSI model. It does operate with multiple signals.

https://en.wikipedia.org/wiki/Multiplexer