According to Piaget, during which stage of cognitive development do the developmental phenomena of object permanence and stranger anxiety occur?

According to Piaget when a child is between 8-9 months of age object permanence emerges.

Explanation:

According to Piaget when a child is between 18-24 months of age object permanence emerges. Object Permanence is the ability in a child where he can symbolically imagine things that cannot be seen at present. For example, if we hide a toy or take it away the child would search for the object understanding the object permanence.