Concept:Piaget’s concrete operational stage (ages 7–11) enables children to solve problems mentally, but only with real, tangible materials—not with abstract symbols.Explanation:In this stage, children can perform mental operations like reversibility (e.g., understanding that if 4+2=6 then 6−2=4) and classification by one feature.They can also seriate objects (e.g., A
However, their thinking remains tied to concrete objects and experiences.They cannot yet handle purely symbolic or hypothetical problems, which develop only in the formal operational stage.Thus, the given description—solving problems mentally but only with materials, not symbols—directly refers to the concrete operational stage.Answer:B. concrete operational