Question

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 1

the answer to this question is the conditioned stimulus.
the conditioned stimulus. refers to an occurence when after becoming associated with a certain stimulus (in this situation, it's the Clown wall paper), this stimulus will become a trigger to a certain conditioned response that the person felt during that time.

Answer 2

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.