Correct statement about photo-chemical smog is

Photochemical smog, also called Los Angeles smog, is formed in warm, dry, and sunny climates (not humid). It is an oxidizing smog, in contrast to reducing smog (London smog), which contains SO$$_2$$ and occurs in humid, cool climates.

It is formed by the action of sunlight on nitrogen oxides and unsaturated hydrocarbons (from automobile exhausts). The key reactions involve NO$$_2$$, O$$_3$$, and hydrocarbons producing formaldehyde, acrolein, and PAN (peroxyacetyl nitrate).

Among the options provided, A states it occurs in a humid climate — incorrect, as it occurs in warm, dry, sunny climates; B describes it as a mixture of smoke, fog, and SO$$_2$$ — incorrect, as that is classical (London/reducing) smog; C claims it is reducing smog — incorrect, since it is an oxidizing smog; D attributes it to the reaction of unsaturated hydrocarbons — correct, as unsaturated hydrocarbons react with NO$$_x$$ in the presence of sunlight to form photochemical smog.

Option D is the correct answer.