In which of Piaget's stages would a child be who has just developed the ability to conserve.

Concrete operational

Explanation:

Piaget was a psychologist who developed a theory on cognitive development from birth to adolescence according to which people go through different stages in their process of thinking developing a more mature and rational thinking as they grow.

One of the stages of this development is called the Concrete Operational stage and it takes place during the ages of 7 to 11. During this stage children start using logic as a way of thinking and start realizing how other people feel and view different situations (and therefore they are less egocentric). One of the main milestones of this period is the development of the concept of conservation: They start understanding that when we take some liquid from a container and move it to a different container, the amount of liquid remains the same, for example.

Therefore, it is in the Concrete Operational stages where a child would have just developed the ability to conserve.