When a billboard has a flashing light (a light on the billboard is flashing, the words themselves are not flashing) that gets you to read the message on the billboard, the flashing light is an example of what type of stimulus prompt?

Within-stimulus promt        

Explanation:

Within-stimulus prompt: In psychology, the term "within-stimulus prompt" is defined as a phenomenon that modifies something that is being related to the stimulus. In within-stimulus prompt, a specific modification is being made to reach out or to identify that change if necessary or it can be detectable in the middle of several other things.

Example: An old man's daughter tends to color his medicine bottles with different colors in order to make him remember which bottle to take in the morning, afternoon, and evening.

In the question above, the given statement signifies "within-stimulus prompt".