The correct answer is A. Excretion.
The kidneys are two bean-shaped organs located in the lower back, just above the waist. They are responsible for filtering waste products from the blood and producing urine. Urine is a waste product that contains water, salt, and other waste products that the body no longer needs. The kidneys also help to regulate blood pressure, produce red blood cells, and maintain a healthy balance of electrolytes in the body.
Respiration is the process of exchanging oxygen and carbon dioxide between the body and the environment. This process takes place in the lungs.
Digestion is the process of breaking down food into smaller molecules that the body can absorb and use for energy. This process takes place in the stomach and small intestine.
Transportation is the process of moving blood, nutrients, and oxygen throughout the body. This process takes place in the heart and blood vessels.