Answer: Conditioned stimulus
Step-by-step explanation:
Conditioned stimulus, in terms of classical conditioning is defined as the priory neutral stimulus that got triggered after coming in contact with unconditioned stimulus to generate conditioned response.
According to the question,tongue clicking is the conditioned response that was neutral but after getting associated with tickling it triggers a response of giggle in Mabel.Thus, Mabel learned the triggering effect produced by tongue clicking.