$[Fe(CN)_6]^{4-} \rightarrow [Fe(CN)_6]^{3-} + e^{-1}\, ;$ $ E^o = -0.35\, V$
$Fe^{2+} \rightarrow Fe^{3+} + e^{-1}\ ;$ $E^o = -0.77\, V$
The substance which has lower reduction potential are stronger reducing agent while the substances which have higher reduction potential are a stronger oxidising agent.
\(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}+\mathrm{e}^{-} \rightarrow\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4} ; \mathrm{E}^{\circ}=0.35 \mathrm{V}\)
\(\mathrm{Fe}^{3+}+\mathrm{e}^{-} \rightarrow \mathrm{Fe}^{2+} ; \quad \mathrm{E}^{\circ}=0.77 \mathrm{V}\)
The reduction potential of \(\mathrm{F} \mathrm{e}^{3+/} \mathrm{Fe}^{2+}\) is higher, hence, \(\mathrm{Fe}^{3+}\) is a strongest oxidising agent.
$(i)\, A3^-\rightarrow A^{2-} + e; E° = 1.5 \,V$
$(ii) \,B^{+}+ e \rightarrow B; E° = 0.5 \,V$
$(iii)\, C^{2+} + e \rightarrow C^{+}; E°= 0.5\, V$
$(iv)\, D \rightarrow D^{2+}+ 2e; E° = -1.15\, V$