The correct answer is D) the achievement of object permanence marks the end of Piaget's sensorimotor period.
Piaget's sensorimotor period is the first stage of his cognitive development theory, which spans from birth to approximately 2 years of age. During this stage, infants explore and understand the world through their senses and motor actions. Object permanence is the understanding that objects continue to exist even when they are not directly perceived.
In the earlier stages of the sensorimotor period, infants lack object permanence and believe that objects cease to exist when they are out of sight. However, as they develop, they begin to understand that objects have a continued existence, even when they are not currently visible. The attainment of object permanence is a significant milestone in cognitive development and marks the end of the sensorimotor period.