Which one of the following psychologists suggested that only observable behaviours should be studied?

Answer: Option A

In the field of psychology, the psychologist who suggested that only observable behaviors should be studied is Watson.

John B. Watson, an American psychologist, is best known for his contributions to behaviorism. He believed that psychology should focus solely on observable behaviors, rejecting introspection and mental processes as valid subjects of scientific study. Watson emphasized the importance of studying behavior objectively, as it could be directly observed and measured.

Therefore, Option A: Watson is the correct psychologist who suggested that only observable behaviors should be studied.