Box B, because he has established object permanence.
The passage explains that infants continue to look under Box A even after seeing the experimenter put the toy under Box B because that is where they are used to finding it and do not realize that the object the experimenter has put out of sight continues to exist in the same location. Knowing that even though the toy is no longer in sight, it must still exist under Box B, the three-year-old will look under Box B.
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What is object performance?</h3>
Object permanence is the understanding that objects continue to exist even when they cannot be sensed. This is a fundamental concept studied in the field of developmental psychology, the subfield of psychology that addresses the development of young children's social and mental capacities. There is not yet scientific consensus on when the understanding of object permanence emerges in human development.
Object permanence is acquired by the end of the sensorimotor stage, that is, by roughly two years of age. A normally developing three-year-old would be expected to have established object permanence.
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