Step 1: What the question wants.
We need the pollutant that gives headache and blurred vision at low levels and can kill at high levels.
Step 2: The danger of this gas.
The gas that fits is carbon monoxide. It is colourless and odourless, so people often breathe it without noticing.
Step 3: Why it harms the body.
Carbon monoxide binds to haemoglobin far more strongly than oxygen does. This blocks the blood from carrying oxygen, so the tissues are starved of oxygen, a state called hypoxia.
Step 4: How the symptoms appear.
With a little oxygen shortage the person gets headache and blurred vision. With a big shortage the brain and heart fail and death can follow. This exactly matches the question.
Step 5: Why the other gases are wrong.
Sulphur dioxide and nitrogen oxides mainly irritate the lungs and cause acid rain. Carbon dioxide chiefly causes the greenhouse effect. None of them block haemoglobin the way carbon monoxide does.
Step 6: Conclusion.
Therefore the correct answer is "Carbon monoxide". \[ \boxed{\text{Carbon monoxide}} \]