Explanation:

Mechanism of the reaction is
\({\mathrm{2 \mathrm{I}^{-}+2 \mathrm{Fe}^{3+} \rightarrow \mathrm{I}_{2}+2 \mathrm{Fe}^{2+}}}\)
\({\mathrm{\left(\mathrm{Fe}^{+3}\right.}}\) oxidises \({\mathrm{\mathrm{I}^{-}}}\)to \({\mathrm{\left.\mathrm{I}_{2}\right)}}\)
\({\mathrm{\mathrm{S}_{2} \mathrm{O}_{8}^{2-}+2 \mathrm{Fe}^{2+} \rightarrow 2 \mathrm{Fe}^{3+}+2 \mathrm{SO}_{4}^{2-}}}\)
\({\mathrm{\mathrm{Fe}^{2+}}}\) reduces \({\mathrm{\mathrm{S}_{2} \mathrm{O}_{8}^{2-}}}\) into \({\mathrm{\mathrm{SO}_{4}^{2-}}}\).
So, the correct option is (4).