Final answer:
A Wig Wag is historically known as a type of railroad crossing signal with a swinging arm and flashing lights to alert of an approaching train.
Step-by-step explanation:
The term Wig Wag can refer to different things in various contexts, but in relation to the options provided, it is closest to part of the stop light system. Historically, a Wig Wag refers to a type of railroad crossing signal that features a swinging arm with lights that flash and swing in a pendulum-like motion to warn of an approaching train. This mechanical arm served as an early warning system at train crossings before the advent of more modern signals. While the term may also allude to other types of signaling devices or even playful motions, in the context of traffic and transportation, it is associated with railroad safety.