The correct answer is the biopsychosocial approach.
The biopsychosocial approach refers to an approach of understanding a behavior or disorder (such as depression) in terms of the biological, psychological as well as social determinants that cause the behavior. In this instance, Dr.McIntosh is adopting the biopsychosocial approach since he believes that depression involves the interactive influences of self-focused negative thinking (psychological factor), rejection from others (social factor), and low serotonin levels (biological factor).