Whenever Tommy Has a Babysitter and Asks to Stay Up

Whenever Tommy has a babysitter and asks to stay up past his bedtime,the babysitter lets him.Whenever Tommy asks his parents to let him stay up past his bedtime,they do not let him.As a result,Tommy only asks the babysitter if he can stay up past his bedtime.The presence of the babysitter at bed time is referred to as a(n) __________ for asking to stay up late:​

A) ​stimulus class
B) ​S-delta
C) ​reinforcer
D) ​discriminative stimulus