Answer:
Respiratory.
Explanation:
The respiratory system helps in the exchange of gases between the environment, lungs and body tissue. The oxygen is inhaled and carbon dioxide is exhaled out during the process of respiration.
Lungs are the important part of the respiratory system. Lungs contain small sac like structures called alveoli, that are the main site of exchange of gases in the body of mammals.
Thus, the correct answer is option (D).
