Photochemical smog is a type of air pollution that is characterized by a mixture of pollutants including ozone, nitric oxide, and unburnt hydrocarbons. It is typically formed when sunlight acts on these primary pollutants.

Let's examine the given options to determine the primary reactants of photochemical smog:

1. Sulphur Hexafluoride and Soot

   This option is incorrect. Sulphur hexafluoride is a gas used as an insulator in high-voltage equipment, and soot is particulate matter. They are not primary contributors to photochemical smog.

2. Carbon Monoxide and Ozone

   Although ozone is a component of photochemical smog, carbon monoxide is not a direct primary reactant in the formation of photochemical smog. Therefore, this option is incorrect.

3. Nitric Oxide and Unburnt Hydrocarbons

   This option is correct. Nitric oxide (NO) and unburnt hydrocarbons from vehicle emissions react in the presence of sunlight to form photochemical smog. The reaction involves complex chemical processes leading to the production of secondary pollutants like ozone.

4. Peroxy Radicals and Sub-oxides

   This option is incorrect. Although peroxy radicals are formed during the reactions leading to smog, they are not the primary reactants. Sub-oxides are not typically involved in the formation of photochemical smog.

Thus, the primary reactants of photochemical smog are Nitric Oxide and Unburnt Hydrocarbons.