Final answer:
A child who is able to use a systematic, scientific problem-solving approach is in Piaget's fourth stage: formal operational thought.
Step-by-step explanation:
According to Piaget's stages of cognitive development, a child who is able to use a systematic, scientific problem-solving approach is in Piaget's fourth stage: formal operational thought. This stage occurs from around age 11 to adulthood. In the formal operational stage, children can think abstractly, handle hypothetical situations, and use logical reasoning to problem-solve. They can consider different perspectives and test alternative solutions. This stage represents the highest level of cognitive development according to Piaget.