What correctly describes the process of expiration?

Expiration, or exhalation, is primarily a passive process during normal quiet breathing. It occurs when the diaphragm and intercostal muscles relax after inhalation, resulting in a decrease in thoracic volume. This reduction in volume creates a pressure difference between the inside of the thoracic cavity and the external environment, causing air to flow out of the lungs naturally due to the elastic recoil of the lung tissues.

While expiration can become an active process during vigorous activities or respiratory distress (where muscles such as the abdominal muscles can be engaged), in typical resting conditions, it happens without the direct use of muscular energy, categorizing it as passive. Most breathing is involuntary, controlled by the autonomic nervous system, though it can also be influenced voluntarily during activities such as speaking or singing.

Considering these factors, "passive process" accurately describes the typical physiological mechanism of expiration during regular, relaxed breathing.