Final answer:
Charlie is in the preoperational stage of cognitive development because he cannot grasp the concept of conservation and believes that the medicine in a larger cup is less than in a syringe.
Step-by-step explanation:
The nurse is identifying that Charlie is in the preoperational stage of cognitive development, which is the second stage in Piaget's theory, covering the ages from approximately 2 to 7 years old. During the preoperational stage, children are unable to understand the concept of conservation, meaning they do not grasp that quantity remains the same despite changes in shape or appearance. Charlie's request to drink the medicine from a 1 ounce cup instead of a 5 mL syringe under the impression that it contains 'less medicine' is a hallmark of this stage, as he's focusing on the immediate, visible volume rather than the actual measurement which requires an understanding of conservation.