Answer:
Conditioned Stimulus: Loud Bang
Step-by-step explanation:
The infant begins to laugh and cough reflexively when he hears the bang, even before he sees his brother. The loud bang is the conditioned stimulus which has been created or associated with greeting the infant with a tickle. The infant has been conditioned to respond to this stimulus.
A conditioned stimulus is created when we learn to react to something because we associate it with something else. It's part of the psychological theory of classical conditioning.