Cory's dad hides his keys under her blanket. cory quickly retrieves and starts playing with them. what piagetian stage has cory mastered?

The answer to this question is object permanence

object permanence refers to the understanding that an object will still remain in a place even though we stop observing it directly.

This psychological phenomenon is the one that make children think their parents is disappearing when they're playing peek-a-boo.