Which of the following devices can be used for switching the output of a discrete DC output module?

A) Transistor.
B) Triac.
C) Relay.
D) Either transistor or relay.

Both a transistor and a relay can be used for switching the output of a discrete DC output module. The choice between the two will depend on factors such as the current and voltage rating of the output module and the load being switched. Therefore, either a transistor or a relay can be the best choice depending on the specific application requirements.