Answer: The correct answer for the blank is -
comparative behaviorist.
Explanation:
Comparative psychology can be described as the scientific study of the behavior and mental processes, particularly relating to the development of behavior (which is drived by rewards and learning), adaptive significance, and phylogenetic history of organisms.
Comparative behaviorist are the researchers that are devoted for such work.
Thus, the given statement, "The major driving forces of behavior are learning and rewards" is made by a comparative behaviorist.