Concept:Understanding Piaget's stages of cognitive development, specifically the stage where conservation is understood.Explanation:Jean Piaget's theory describes distinct stages of cognitive development in children. The ability to understand the 'law of conservation' – the idea that quantity remains the same despite changes in appearance – is a key milestone. This understanding typically develops during the Concrete operational stage, which spans from about seven to eleven years old. During this stage, children can think logically about concrete events and understand concepts like reversibility and decentration, which are crucial for grasping conservation. The other stages, Sensory-motor, Pre-operational, and Formal operational, do not primarily characterize the understanding of conservation.Answer:C. Concrete operational stage