Final answer:
The correct answer is c) Concrete operational. During this stage, a child develops the ability to perform mental operations on concrete objects and understands conservation and reversibility.
Step-by-step explanation:
Once a child can perform mental operations on concrete objects and understands the principles of conservation and reversibility, she has reached Piaget's concrete operational stage. This is Piaget's third stage of cognitive development, occurring from about 7 to 11 years old. In this stage, children can think logically about concrete events and can grasp the use of numbers, understand mathematical operations, and manage memory strategies.
They also develop a clear understanding of the idea of conservation, recognizing that the quantity of a substance remains the same even when its appearance changes, as in the example of water being poured between glasses of different shapes.
Furthermore, children at this stage grasp the principle of reversibility, which allows them to understand that objects can change form and return to their original condition without their quantity changing.