Physical barrier provides the first line of defense to our body. They do not allow microbes to invade into our body. These include skin and mucous membrane. Skin does not allow the microorganism to penetrate into the blood, Skin secretes acidic sweat and other secretions that contain hydrolytic enzymes that do not allow microbes to grow.
Coughing and sneezing are also the part of physical barrier because through coughing and sneezing the microbes are thrown out of the body by physical process. Cough includes many mucus entrapped bacteria in it. Therefore the right answer is d. physical.
