Final answer:
Kerry's inability to understand that his father can also be his grandfather's son is indicative of egocentrism, which suggests he is in the preoperational stage of cognitive development, according to Piaget's theory.
Step-by-step explanation:
The scenario involving Kerry's conversation with his grandfather strongly suggests that Kerry is in the preoperational stage of cognitive development, according to Jean Piaget's theory. This developmental stage typically occurs between the ages of 2 and 7 years. In the preoperational stage, children exhibit characteristics such as symbolic thinking, pretend play, and a developing but still limited use of language.
One notable aspect of the preoperational stage is egocentrism, where children struggle to understand perspectives other than their own. Kerry's inability to grasp that his father can also be someone else's son is a clear manifestation of egocentrism. This difficulty in comprehending others' viewpoints is a hallmark of the preoperational stage. Therefore, the accurate response to the student's question about Kerry's cognitive development is indeed a) Preoperational stage.