The correct answer is C, “±10 V.”

In order to indicate both direction and velocity, a signal with both positive and negative characteristics is required. Motion controls use either ±10 V or ±5 V signals to accomplish this. Direction is indicated by the sign of the signal; magnitude of velocity is indicated by the magnitude of the voltage.

The other three choices are typical signal types for analog inputs, but none have the ability to easily indicate direction, only velocity. Obviously, a pneumatic (3–15 psi) signal would never be a good choice for motion control applications.

Reference: Trevathan, Vernon L., A Guide to the Automation Body of Knowledge, Second Edition, ISA, 2006.