Answer:
Residual volume.
Explanation:
Residual volume is the volume of air that always present in the lungs and cannot be expelled even after forceful expiration. The average residual volume remains in the lungs is 1200ml.
Residual volume is used by the doctors to determine the lung functioning. The residual volume prevents the lungs from collapsing in case of any disease or absence of air.
Thus, the correct answer is option (a).