Answer: Stimulus generalization
Explanation: stimulus generalization occurs when an individual responds to a stimulus in a way similar to that which has been learnt from a previous stimulus of similar nature. Stimulus generalization occurs when the same or similar stimulus evokes the same response from an individual.
In the scenario above, Ashley's fear for clowns started at the age of three after being startled by the appearance of clowns. His response to that stimulus didn't change though as he still habors that response(fear) whenever he sights anyone with similar stimulus ( anyone in costume and mask).