Answer:
The concrete operational stage
Step-by-step explanation:
The child presented with the two balls will be able to correctly solve the problem in the concrete operational stage: A child shows this ability to "preserve" or conservate things mentally.
The conservation is a mental process that involves exposure to situations like watching the two balls, where one remains in its original form while the other is rolled, and thus being able to mentally manipulate the object.
In concrete operational phase, child is able to think logically, following rules. It is also characterized by overcoming the inadequate mental operations present in the previouvs phase- the preoperational.
In Concrete operational thinking children are in the third stage of cognitive development, Piaget believes children in this stage follow logical reasoning in situations where changes in appearances are present and yet they are able to solve the problems.