Answer:
c) conditioned stimulus.
Step-by-step explanation:
Conditioned stimulus: The term conditioned stimulus is a part of the classical conditioning, which is proposed by the behavioral psychologist named Ivan Pavlov. It is defined as a formerly neutral stimulus that after being associated with an unconditioned stimulus triggers a conditioned response. Here, the neutral stimulus doesn't produce any reaction.
In the question above, the sound was a conditioned stimulus.