D

Hypoxia is a cardinal manifestation of respiratory failure [Roussos & Koutsoukou 2003, PMID 14621112]; inability to speak in full sentences is a recognized indicator of severe respiratory distress [Shapiro et al. 2005, PMID 16147481]; and inspiratory stridor signals upper-airway obstruction leading to respiratory compromise [Leung & Cho 1999, PMID 10593320]. Because A, B and C are all true signs of respiratory insufficiency, the only option that does not represent such a sign is “All of the above,” making D the correct choice.