The concept of object permanence is attained during Piaget's _____ stage of development.

According to Piaget's theory of cognitive development, object permanence—the understanding that objects continue to exist even when they cannot be seen, heard, or touched—is attained during the sensorimotor stage (birth to 2 years). This is one of the most important achievements of this stage, typically developing around 8-12 months. Before this, infants behave as if hidden objects no longer exist. This milestone marks a significant cognitive development in the child's understanding of the world.