What is the term for air in the pleural cavity?

The term for air in the pleural cavity is pneumothorax.

The pleural cavity, more precisely called the pleural space, lies between the visceral pleura covering the lung and the parietal pleura lining the chest wall. It normally contains only a very small amount of lubricating fluid. When air enters this space, the normal pressure relationship that keeps the lung expanded is disrupted, and part or all of the lung may collapse.

A pneumothorax can occur spontaneously, after chest trauma, or as a complication of a medical procedure. A primary spontaneous pneumothorax happens without known major lung disease; a secondary one occurs in people with conditions such as emphysema or tuberculosis. Smoking increases the risk of spontaneous cases.

The wrong options describe different substances in the same space: hydrothorax is fluid, hemothorax is blood, and chylothorax is lymphatic chyle. A tension pneumothorax is a dangerous form in which pressure rises progressively and can impair breathing and circulation.