Answer: Variable
Explanation:
Variable is the factor that as the tendency to get modified or varied as per condition. There are two types of variable in research- independent variable and dependent variable .
According to the question,Bertina's experiment includes intelligence as the variable because this factor is varying as per age of people.
Other options are incorrect because sample, confound and constant is not represented through intelligence factor in Bertina's research.Thus, the correct option is variable.
