Answer: Object permanence
Explanation: Object permanence is the mechanism that describes about the fact that the objects and things will exist and events will take place in environment even though if it cannot be sensed or perceived.
Erick does not display the quality of understanding the object permanence.He is interested in the keys till he can see it and would cry for it when its taken away from it.
As soon as keys are hidden from him,he gets involved into something else and stop crying thinking that it gone or doesn't exist anymore .Thus, he is not aware that keys exist in the hidden manner.