Answer:
Conservation.
Step-by-step explanation:
Jean Piaget was a psychologist who developed a cognitive theory in children and adolescents according to which our thinking goes through different stages as we get older and it becomes more complex and logic in every stage.
One of the terms of Piaget's theory is the term of Conservation which refers to the understanding that kids develop when they realize that something stays the same regarding quantity even though the way it is presented might change. In other words, the same amount of a thing is not affected if we present it in two different ways.
In this example, one child begs for more than one cookie for snack, so her mother breaks the cookie into two pieces. The child is still upset because he realizes that even though there are two pieces, the amount of cookie remained the same, and it was not affected just because it is being presented differently. Therefore, this child is demonstrating an understanding of Conservation.