Bob has an unknown liquid and must determine what it is.He decides to systematically use a number of tests to determine the correct answer.According to Piaget, Bob has most likely reached the _____ stage of cognitive development.

A) sensorimotor
B) preoperational
C) formal operational
D) object permanence

The formal operational stage is marked by the ability to think logically and systematically and to develop hypotheses and test them out using deductive reasoning. Bob's approach to determining the nature of the unknown liquid demonstrates this type of systematic, logical thought process. The sensorimotor stage is characterized by exploration through the senses and movements, while the preoperational stage is marked by the emergence of symbolic thought but not systematic, logical thought. Object permanence is a concept developed in the sensorimotor stage related to the understanding that an object still exists even when it is no longer visible.