Six-year-old kian is in the stage of cognitive development, whereas his 10-year-old brother is likely in the stage.

preoperational; concrete operations                

Explanation:

Jean Piaget has given the theory of cognitive development in which he has mentioned four different stages including sensorimotor, preoperational, concrete operational, and formal operational.

Preoperational stage: This is the second stage in the cognitive development theory and starts from the age of two years and lasts through seven years of age in a child's stage. At this stage, a child is being engaged in manipulating symbols and symbolic play.

Concrete Operational Stage: This stage is the third stage and starts at seven years of age and lasts through eleven years of a child's life. In this stage, a child is engaged in using logical operations or thought and therefore the child can put logic only to physical objects.

your answer is "graduates" hope this