Answer: D. alveoli
Step-by-step explanation:
Respiration is a metabolic chemical process which involves the exchange of gases. It involves the intake and distribution of oxygen from the outer environment to all the body cells of the living beings and the removal of carbon dioxide from all the cells of the body. When the oxygen enters into the lungs it is distributed into tube like structures which are called as bronchioles these bronchioles get split into further network of small tubes called as alveoli. In the alveoli the path of distribution of oxygen opens up to all the parts of the body and the carbon dioxide gas is breath out.