Answer: Hypothesis
Explanation: Hypothesis is the way of presenting the idea by studying on that and evaluating it to present it in the form of theory. This process also includes the experimenting on the ideas so that some conclusion can be obtained.
The case mentioned in the question, hypothesis is the medium through which the psychologist is able to predict the children's belief on Santa Claus according to different age group (E.g.- 5 years,10 years and 15 years ).Thus, psychologist is the taking the help of hypothesis which describes about the mind of children at different age.