Nine-month-old Avery can retrieve his ball when his mother hides it under a blanket. Avery has begun to master __________.
Correct Answer: C
Rationale: Avery demonstrating the ability to retrieve his ball hidden under a blanket indicates that he has started to understand object permanence. Object permanence is the concept that objects continue to exist even when they are out of sight. This milestone typically occurs around 8-12 months of age. The other choices are incorrect. Option A, the tertiary circular reaction, is related to Piaget's theory of sensorimotor development but does not specifically address object permanence. Reflexive schemes (Option B) refer to simple, reflex-like behaviors and are not directly linked to object permanence. Deferred imitation (Option D) involves the ability to imitate actions observed at an earlier time, which is a different developmental milestone compared to object permanence.