Sulphur dioxide (SO₂) is a colourless, pungent gas released mainly by burning sulphur-containing fossil fuels (coal, diesel) and by smelting and other industrial processes. The question asks for the mechanism by which it irritates the upper respiratory tract, and the key clue is that SO₂ is highly water-soluble.

Because it dissolves so readily in water, SO₂ is absorbed by the moist mucous lining of the nose, throat, and upper airways before it can travel deep into the lungs. On contact with this moisture it reacts to form sulphurous acid (H₂SO₃):

SO₂ + H₂O → H₂SO₃

This freshly formed acid is the actual irritant. It inflames the mucous membranes, producing coughing, a burning or scratchy throat, wheezing, and worsening of asthma and bronchitis. This is exactly the option stating that SO₂ dissolves in the moisture of the upper respiratory tract and forms an irritant, so it is correct.

Why the other options are wrong:

• Combining with haemoglobin to affect oxygen transport describes the mechanism of carbon monoxide (CO), not SO₂; SO₂ does not chiefly act by binding haemoglobin.
• Oxidising to sulphur trioxide and being inhaled deep into the lung contradicts the observed behaviour: SO₂'s high water solubility means it is trapped in the upper tract, which is precisely why upper-airway irritation (rather than deep-lung deposition) predominates.
• Combining with other pollutants and polymerising in the lungs is not a real chemical pathway for SO₂; the irritation comes from simple acid formation, not polymerisation.