Step 1: Understanding the Concept:
According to Jean Piaget's theory of cognitive development, children progress through four distinct stages: sensorimotor, preoperational, concrete operational, and formal operational.
Step 2: Detailed Explanation:
- Assertion (A): A nine-year-old child is in the concrete operational stage (which spans from 7 to 11 years). During this stage, children master the principle of conservation (the understanding that physical quantities like mass, volume, and number remain unchanged despite changes in shape or appearance). Thus, they understand that a ball of clay maintains the same mass when flattened. This makes Assertion (A) true.
- Reason (R): A key cognitive milestone of the concrete operational stage is the acquisition of reversibility (the mental capacity to reverse steps and return to a starting point). Preoperational children (2--7 years) struggle with reversibility, but concrete operational children have mastered it. Therefore, the statement that concrete operational children cannot reverse their thinking is false.
Step 3: Final Answer
Since the Assertion is scientifically accurate but the Reason is factually incorrect, (A) is true but (R) is false.