Question

pneumothorax
air in the lungs
blood in the pleural cavity
water in the pleural cavity
collection of air in the pleural cavity

Explanation:

Pneumothorax is defined as the collection of air in the pleural cavity. The first option "air in the lungs" is incorrect as it's not in the pleural cavity. The second option "blood in the pleural cavity" is hemothorax. The third option "water in the pleural cavity" is hydrothorax.

Answer:

collection of air in the pleural cavity