Step 1: Recall the recipe:
Photochemical smog needs three things: nitrogen oxides, volatile organic compounds and strong sunlight.
Step 2: Match the options:
Options (1), (2) and (4) are exactly these three ingredients.
Step 3: Look at the odd one out:
That leaves water vapour in option (3). It is part of foggy classical smog, but the Los Angeles type forms in dry sunny weather.
Step 4: Cross-check with the passage:
The passage lists water vapours only for classical smog, and sunlight with nitrogen oxides and hydrocarbons for the photochemical type.
Step 5: Conclude:
Water vapour is not a must for photochemical smog.
Extra check:
Think of London smog as foggy and Los Angeles smog as sunny and dry. Foggy smog needs water vapour. Sunny smog needs light, nitrogen oxides and hydrocarbons. Since the question is about the sunny type, water vapour is the one item that is not a must. Plants and people suffer from the sunny type too, but it never needs moist air to start.
Final Answer:
The answer is water vapour.
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