Which of the following respiratory volumes measures the amount of air that always remains in the lungs to prevent the lungs from collapsing?
(a) residual volume
(b) expiratory reserve volume
(c) inspiratory reserve volume
(d) tidal volume

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).