Which are the primary constituents of photochemical smog?

The correct option is B NO${}_2$, O${}_3$ and hydrocarbons
Photochemical smog forms when primary pollutants like nitrogen oxides and volatile organic compound react with ultraviolet light to create a variety of toxic and reactive compounds. Ultraviolet light can split nitrogen dioxide into nitric oxide and monatomic oxygen, this monatomic oxygen can then react with oxygen gas to form ozone, aldehydes, and peroxyacetyl nitrates which are the secondary pollutants. The mixture of these primary and secondary pollutants forms photochemical smog. Thus, the correct answer is option C.