Final answer:
Sulfur dioxide (SO2) is NOT one of the principal components required for photochemical smog, which is primarily composed of nitrogen oxides, hydrocarbons, and sunlight. Therefore, correct option is 3.
Step-by-step explanation:
The principal components required for photochemical smog are nitrogen oxides (NOx), hydrocarbons (often referred to as volatile organic compounds or VOCs), sunlight (solar radiation), and typically ozone as a secondary pollutant. Sulfur dioxide (SO2) is not one of the principal components required for the formation of photochemical smog.
Photochemical smog is a complex mixture of air pollutants which includes secondary pollutants such as ozone, formed by the reaction of sunlight with primary pollutants like nitrogen oxides and hydrocarbons emitted by vehicles and industrial sources.