Nine-year-old Jamal watches as someone pours liquid from a short, wide container into a taller, narrower container. He realizes that the amount of liquid is unchanged, indicating that he has reached the stage of cognitive development.

concrete operational.                                                      

Explanation:

Concrete operational stage: In psychology, the term "concrete operational stage" is described as one of the different stages in the cognitive development theory which was introduced by Jean Piaget. This stage starts from seven years of age and lasts through eleven years of age and is considered as the third stage in the theory. At this stage, the child begins to develop logical or rational thoughts and struggles with hypothetical and abstract concepts.

In the question above, the given statement represents the "concrete operational stage" as Jamal has developed logical thoughts.

the answer is:

c. economic output

is there a year set? the question is incomplete