Final answer:
The correct completion for the sentence is A. Residual volume, 1.2 liters. Residual volume is the air remaining in the lungs after maximal exhalation and cannot be directly measured.
Step-by-step explanation:
The answer that most correctly completes the sentence "There is still some air which remains within a person's lungs after he/she has exhaled; that air volume is termed residual volume, and it is around 1.2 liters." is A. Residual volume, 1.2 liters.
Residual volume is the amount of air that remains in the lungs after a maximal exhalation and is crucial for preventing the collapse of alveoli and large fluctuations in O₂ and CO₂ levels. It is the only lung volume that cannot be measured directly because the lungs cannot be completely emptied of air.