Photochemical smog is generated from automobile exhaust when hydrocarbons and nitrogen oxides react in the presence of strong sunlight; it typically appears on sunny days and harms crops, trees, and human health (eye irritation, asthma).

Introduction / Context:
Photochemical smog is a secondary air pollution phenomenon found in urban areas with heavy traffic and strong solar radiation. It involves sunlight-driven reactions of precursors from vehicle exhaust.

Given Data / Assumptions:

• Primary precursors: hydrocarbons (HC/VOCs) and nitrogen oxides (NOx).
• Intense sunlight drives reactions.
• Products include ozone (O3), PANs, and oxidants.

Concept / Approach:
Sunlight catalyzes reactions of NOx and VOCs to form oxidants; these oxidants cause health and vegetation damage.

Step-by-Step Solution:

Verification / Alternative check:
Observationally strongest on clear, sunny, stagnant days; declines with reduced NOx/VOC emissions and after sunset.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing photochemical smog with reducing (London) smog; assuming fog is necessary; overlooking VOC controls.

Final Answer:
All of the above