Sulfonation of benzene with sulphur trioxide in the presence of conc. sulfuric acid gives benzene-1,3-disulphonic acid.Treatment with aqueous NaOH forms disodium salt of benzene-1,3-disulphonic acid. On fusion with solid sodium hydroxide disodium salt of resorcinol is obtained. Acid hydrolysis gives resorcinol.Hence, the correct option is B .