Final answer:
Summer has reached the Concrete Operational stage of Piaget's cognitive development stages, as she understands the concept of conservation.
Step-by-step explanation:
Summer, who is seven years old, demonstrates understanding that a candy bar remains the same size regardless of whether it's in two pieces or six pieces. This ability to understand conservation indicates that she has reached the Concrete Operational stage of Piaget's cognitive development stages. This third stage, according to Piaget, occurs from about 7 to 11 years old and is characterized by the development of logical thinking about concrete situations, as well as grasping the concept of conservation.