Correct statement about smog is

A. Classical smog occurs in cool humid climate. It is a mixture of smoke, fog, and sulphur dioxide. Chemically it is a reducing mixture and so it is called reducing smog. So statement A is incorrect.

B. Classical smog is a mixture of smoke, fog and ${SO}_{2}$. ${NO}_{2}$ is not present in classical smog. So, statement B is incorrect.

C. Photochemical smog occurs in warm, dry and sunny climate. The main components of the photochemical smog result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles and factories. Photochemical smog has high concentration of oxidising agents and is, therefore, called as oxidising smog. So, statement C is correct.

D. Classical smog is a reducing mixture, so it is called as reducing smog. Hence, statement D is incorrect.