What condition is characterized by breathing comfortably only when in an erect sitting or standing position?

The condition characterized by the ability to breathe comfortably only when in an erect sitting or standing position is known as orthopnea. This term specifically describes difficulty breathing when lying flat, which is often relieved by moving into a more upright position. It is commonly associated with heart failure or certain lung conditions, where fluid accumulation in the lungs can lead to increased difficulty in breathing when supine.

Other terms present in the choices refer to different respiratory issues. Dyspnea is a general term for difficulty or labored breathing, while apnea refers to a temporary cessation of breathing. Hypopnea involves shallow or slow breathing but does not specifically relate to positional breathing issues like orthopnea does. Hence, orthopnea is the correct answer as it uniquely identifies the positional nature of the breathing difficulties experienced.