Answer:
Formal operational stage
Step-by-step explanation:
In the formal operational stage of Piaget's theory of the development of cognition a child learns to think of concepts in an abstract manner, also they begin to manipulate and play with ideas in their mind.
The child is able to solve problems using trial and error methods. They are aware of the fact that they are thinking.
Hence, the question is referring to the formal operational stage of development.