Reducing smog is a mixture of:

Smog is classified into two types: reducing smog (classical smog) and photochemical smog (oxidizing smog).

Reducing smog, also called London smog or classical smog, occurs in cool, humid climates and is caused by the burning of coal and other fossil fuels. It is a mixture of smoke, fog, and sulfur dioxide ($$SO_2$$). The $$SO_2$$ in reducing smog can further react with moisture to form sulfuric acid, which is why it has a reducing character and causes acid rain.

Photochemical smog, on the other hand, contains oxidants like ozone ($$O_3$$) and is formed in warm, sunny conditions from nitrogen oxides and hydrocarbons.

Therefore, reducing smog is a mixture of smoke, fog, and $$SO_2$$, which is option (B).