Bella developed a phobia of clowns after she experienced a scary thunderstorm in a room with clown wallpaper as a child. in bella's case the clown wallpaper served as:

Answer is unconditioned stimulus.

In classical conditioning which is learning process the term unconditioned stimulus or in short UCS is used when unconditionally and without a person’s control a response is initiated. For example if someone likes roasted chicken, the smell will automatically trigger hunger.

answer; beware of exaggerated claims without supporting evidence;