The following is a description of a child being assessed for formal operations. "The boy moved the blocks over so that the lever was now free to rotate up and down

When asked about why it now moved, he was able to adequately describe what was taking place. According to this description, it is clear that the child is in which of Piaget's stages of cognitive development?
a. sensorimotor
b. preoperational
c. concrete operations
d. formal operations

D. The child is in formal operations because he can explain the rationale behind the solution.