An aqueous solution of salt containing an acidic radical ${\mathrm{x}}^{-}$ reacts with sodium hypochlorite in neutral medium. The gas evolved produces blue black colour spot on the starch paper. The anion ${\mathrm{x}}^{-}$ is.

${\mathrm{I}}^{-}+{\mathrm{OCl}}^{-}⟶{\mathrm{I}}_2$  (Blue black colour spot on starch paper

${\mathrm{OCl}}^{-}+3{\mathrm{I}}^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{I}}_3^{-}+2{\mathrm{OH}}^{-}+{\mathrm{Cl}}^{-}$

${\mathrm{l}}_3^{-}+\text{starch }\to$Iodine starch adsorption complex causes a blue-black stain to emerge on starch paper. The correct option is C.