Milestone-to-Stage Match: Object permanence is a well-known, specific milestone in Piaget's theory — match it directly to its stage.
The Sensorimotor stage (birth to ~2 years) is when infants learn the world through senses/actions and develop object permanence — understanding objects exist even when unseen.
Preoperational, Concrete operational, and Formal operational stages come LATER and are defined by different milestones (symbolic thought, logical reasoning, abstract thinking) — object permanence is already established by then.