Final answer:
John P. Grey was an advocate for biological psychiatry, the belief that mental disorders have physical causes.
Step-by-step explanation:
The question asks about a great modern development advanced by John P. Grey. John P. Grey advocated for biological psychiatry and opposed psychological explanations for mental illness.
He believed that mental disorders were rooted in physical causes, such as brain pathology, and advocated for more humane treatment of patients with mental illness but did not believe in psychotherapy approaches. Therefore, the correct answer to the question is C) Biological psychiatry.