Answer:
conditioned stimulus
Explanation:
In this specific scenario, tongue clicking is an example of a conditioned stimulus. This refers to a previously neutral stimulus that, after becoming associated with the unconditioned stimulus, eventually comes to trigger a conditioned response. Which in this scenario, Francis is associating the clicking of the tongue (conditioned stimulus) with the tickling (unconditioned stimulus) which now always triggers Francis to squirm and giggle (conditioned response).