What is the most common cause of airway obstruction in unconscious patients?

The correct answer is Flaccid tongue blocking the hypopharynx
The most common cause of airway obstruction in unconscious patients is the flaccid tongue blocking the hypopharynx. The collapse of the upper airway is common in unconscious patients.

Patients with a tendency to upper airway obstruction during sleep are vulnerable when unconscious or sedated.

The loss of wakefulness worsens depression of airway muscle activity, so they (patients) cannot respond adequately to asphyxia.

All other options listed above, such as crush injury to the trachea, foreign body obstruction, and edema of the vocal cords, can cause airway obstruction, mostly inpatient in the conscious state, rather than causing an obstruction in patients in an unconscious state.