Which of the following is NOT a signal light used to guide vehicle drivers?

The correct answer highlights that a flashing yellow light is not typically used as a signal light to guide vehicle drivers in standard traffic contexts. In traffic control, steady red and green lights are essential because they denote stop and go signals, respectively. A steady red light instructs drivers to stop, while a steady green light allows them to proceed safely.

The flashing yellow light, on the other hand, is primarily used to signal caution, often indicating that drivers should slow down and be prepared to yield, especially in areas where the conditions may change or where there may be pedestrians or construction work. It is not a directive that explicitly instructs drivers to stop or go like the steady lights do.

Flashing white lights are also used in specialized scenarios, such as in emergency or construction contexts, but they are not part of the common traffic signal system for guiding drivers. Thus, the flashing yellow light stands out as a non-standard signal in regular traffic management, making it the correct answer to the question.