Answer:
c.Observational study. The variable of interest is the result of either correct or incorrect to each trial.
Explanation:
Observational study is the type of study when a researcher don't influence the outcome of an experiment by imposing a treatment and only observe the outcome of an experiment. A nine year old tested professional observes therapists choices and so this is the observational study.
A nine year old tested professional observes therapists choices and the choice of therapist can be correct or incorrect. So, the variable of interest is the result of either correct or incorrect to each trial.