d
\(\mathrm{KMnO}_{4}\left(\mathrm{Mn}^{7+}\right)\) changes to \(\mathrm{Mn}^{2+}\) l.e., number of electrons involved per mole of

\(\mathrm{KMaO}_{4}\) is \(5\)

(a) For \(\mathrm{FeSO}_{3}\) \(\left.\mathrm{Fe}^{2+} \longrightarrow \mathrm{Fe}^{3+} \quad \text { (No. of } e^{-} \mathrm{s} \text { involved }=1\right)\)

\(\mathrm{SO}_{3}^{2-} \longrightarrow \mathrm{SO}_{4}^{2-} \quad\left(\mathrm{No} . \text { of } e^{-} \mathrm{s} \text { involved }=2\right)\)

Total number of \(e^{-}\) s involved \(=1+2=3\)

(b) For \(\mathrm{FeC}_{2} \mathrm{O}_{4}\)

\(\left.\mathrm{Fe}^{2+} \longrightarrow \mathrm{Fe}^{3+} \quad \text { (No. of } e^{-\mathrm{s}} \text { involved }=1\right)\)

\(\left.\mathrm{C}_{2} \mathrm{O}_{4}^{2-} \longrightarrow 2 \mathrm{CO}_{2} \text { (No. of } e^{-\mathrm{s}} \text { involved }=2\right)\)

Total number of \(e^{-}\) s involved \(=1+2=3\)

(c) For \(\mathrm{Fe}\left(\mathrm{NO}_{2}\right)_{2}\)

\(\left.\mathrm{Fe}^{2+} \longrightarrow \mathrm{Fe}^{3+} \quad \text { (No. of } e^{-} \text {s involved }=1\right)\)

\(\left.2 \mathrm{NO}_{2} \rightarrow 2 \mathrm{NO}_{3}^{-} \text {(No. of } e^{-} \text {s involved }=4\right)\)

Total number of \(e^{-s}\) involved \(=1+4=5\)

(d) For \(FeSO_4\), \(\left.\mathrm{Fe}^{2+} \longrightarrow \mathrm{Fe}^{3+} \quad \text { (No. of } e^{-} \mathrm{s} \text { involved }=1\right)\)

Total number of \(e^-\) 's involved \(= 1\)

As \(FeSO_4\), requires least number of electrons thus, it will require least amount of \(\mathrm{KMnO}_{4}\)