The inability to breathe unless in an upright position is:

A) apnea.
B) eupnea.
C) dyspnea.
D) orthopnea.

Orthopnea

Difficulty breathing that occurs when lying down and is relieved by sitting up, often associated with heart failure.

Breathing

The process of taking air into and expelling it from the lungs, essential to supply the body with oxygen and remove carbon dioxide.

Upright Position

A body posture where an individual stands or sits straight, with the spine aligned vertically.