Final answer:
Walter Dill Scott's primary contribution to industrial-organizational psychology was applying psychology to advertising, management, and personnel selection (option A).
Step-by-step explanation:
Walter Dill Scott, one of the early pioneers in the field of industrial-organizational psychology, made significant contributions by applying psychology to advertising, management, and personnel selection.
He was among the first psychologists to recognize the value of psychology in understanding and improving the business world.
Scott published several books on the use of psychology in business, covering topics such as advertising, employee selection, and improving efficiency in the workplace.