Answer:
Biological predisposition of learning
Step-by-step explanation:
There are some inbuilt likeliness towards doing a work even when subjected to external stimuli. This tendency is known as Biological predisposition of learning. A person (here the baby) will wave the banner only when he/she is exposed to desired stimuli. Since the parents are not aware of those desired stimuli conditions, they are unable to make the baby wave the banner even though he/she picks up the banner.
Thus, "Biological predisposition of learning" is keeping the baby from learning to wave the banner