Of late, Steve has been feeling lethargic and frustrated. He is a moderate smoker who never exercises. He visits his doctor, and the doctor observes that he has also been extremely stressed recently due to work pressure. The doctor then suggests that Steve makes some lifestyle changes that include changes in his diet and also an exercise regimen that he has to strictly adhere to. On the basis of these characteristics, which of the following models explaining the correlation of stress and illness is Steve's doctor employing?