The correct answer is D) complete obstruction of respiratory passages. Severe airway burns can cause significant damage and swelling to the respiratory tract, leading to a complete obstruction of the airway. This is a life-threatening condition that requires immediate medical attention. The other options, such as reddening of the cheeks (A), nosebleed (B), and nausea (C), are not directly caused by severe airway burns and do not represent the most serious potential consequence.