Explanation

CO is correct because it binds to hemoglobin with much higher affinity than oxygen, causing functional anoxia. SO2 is incorrect because while toxic, CO is the primary gas listed known for causing classic anoxia symptoms via binding. CO2 is incorrect because high levels of CO2 cause respiratory acidosis but are not the direct cause of anoxia in this context. NO2 is incorrect because it primarily causes irritation and respiratory distress rather than pure anoxia like carbon monoxide.