Photochemical smog is a type of smog produced when ultraviolet light from the sun reacts with nitrogen oxides in the atmosphere. It is visible as a brown haze, and is most prominent during the morning and afternoon, especially in densely populated, warm cities

The main components of photochemical smog are nitrogen oxides, Volatile Organic Compounds (VOCs), tropospheric ozone, and PAN (peroxyacetyl nitrate).

Among the following options, NO₂, O₃ and unsaturated hydrocarbon are components of photochemical smog where they required a warm, dry and sunny climate to get oxidized. SO₂, on the other hand, has a reducing nature. Hence cannot be the component of photochemical smog.