You show a child a toy and then hide that toy behind a chair, and when the child wants the toy, he or she only looks in plain sight, not behind the chair. according to piaget, the child could be said to lack a mastery of

Object permanence

Explanation:

Object permanence is part of the sensori-motor stage. The sensori-motor stage is the first stage of cognitive development theory of Piaget. In this theory, Piaget discusses the cognitive development of children. Object permanence is the basic part of the first stage. In this process, the children know about the physical stability of the objects even when the object is not in front of their eyes. They believe if object ins not in front of their eyes but still the object does exist.  

Stages to development of the object permanence:

a.indigo and cochineal

b.animal hides

c.silver and gold

e.cacao beans and sugarcane