The correct answer is psychologist.
There are several differences between the professions of a psychologist and a psychiatrist. The difference is in the nature of the treatment provided by both of them.
Psychiatrists are trained medical doctors and they can prescribe medications.
Psychologists concentrate their work on psychotherapy and and solving mental and emotional issues of their patients.
Dr. Williams is not able to prescribe medication because he is a psychologist and not a psychiatrist.