The correct answer is the preoperational stage of cognitive development.
According to Piaget's theory of cognitive development, children in the preoperational stage have not developed the ability of conservation. This means that children do not understand that the quantity of something remains the same despite changes to its arrangement or type of container it is placed in. In this instance, children did not understand that the quantity of water poured from a beaker to a taller, narrower one remained the same.