You tried to play peek-a-boo with your 1-month-old nephew, but whenever you hid your face, he would cry. you try it again 6 months later, and now he laughs and enjoys the game, trying to uncover your face with his hands. your nephew has begun to develop t

Object permanence.

Explanation:

Object permanence may be defined as the understanding of the objects without touching and seeing them. This includes the development of social and mental capacities.

The child uses the object permanence after seeing their known person. Earlier, the nephew cries after hiding the face but now he enjoys it. This is because of his mental, social and physical development. The object permanence generally takes place in case of infants.

Thus, the answer is object permanence.