Final answer:
A child lacking the understanding of object permanence is in the sensorimotor stage, which is the initial phase of Jean Piaget's cognitive development stages. This stage lasts from birth to approximately 2 years old and is characterized by learning through sensory and motor interactions.
Step-by-step explanation:
A child who has not yet developed object permanence is in the sensorimotor stage of Piaget's theory of cognitive development. This stage spans from birth to about 2 years of age, during which children learn about the world primarily through their sensory experiences and motor actions.
It is in the sensorimotor stage that infants gradually begin to realize that objects continue to exist even when they are out of sight, a concept known as object permanence. Prior to this development, infants will not search for objects that have been hidden, as they do not yet understand that these objects still exist. As children progress through the sensorimotor stage, they gain this critical understanding.
A child who does not understand object permanence is in the sensorimotor stage (a). This stage of cognitive development is characterized by the exploration of the world using senses and motor behavior, and precedes the development of object permanence.