Answer: a) "pull" of external stimuli.
Step-by-step explanation:
Pull of external stimuli is indicated through making stimuli that makes a goal more attractive .This motivates a person towards something through external factors or element
According to the question, pull of external stimuli is seen as pie is acting as the external stimuli which was earlier not a goal but due to the attraction it had Stefani's focus on it , it made her eat pie even after large meal.
Other options are incorrect because push of external stimuli, homeostasis process and decrease in motive of stimulus is not the cause of Stefani having large pie after huge meal.Thus, the correct option is option(a).