ANS:C - both 'a' & 'b'

Both nitrogen oxides (NOx) and hydrocarbons (HC) in the exhaust gas of an internal combustion (I.C.) engine contribute to the formation of photochemical smog. Therefore, the correct option is: both 'a' & 'b' Photochemical smog is formed when nitrogen oxides and hydrocarbons react in the presence of sunlight to produce secondary pollutants such as ozone and peroxyacetyl nitrate (PAN). This reaction is facilitated by the presence of sunlight and leads to the formation of the characteristic brown haze associated with smog. Nitrogen oxides and hydrocarbons are primary pollutants emitted by vehicles and other sources, and their presence in the atmosphere contributes significantly to the formation of photochemical smog.