Identify the reaction from following having top position in EMF series (Std. red. potential) according to their electrode potential at 298 K.

$\begin{array}{l}{\mathrm{Au}}^{3+}+3{\mathrm{e}}^{-}\to \mathrm{Au}\left(\mathrm{s}\right)\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}{\mathrm{E}}^{\circ }=1.40\mathrm{V}\\ {\mathrm{Fe}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{Fe}\left(\mathrm{s}\right)\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}{\mathrm{E}}^{\circ }=-0.44\mathrm{V}\\ {\mathrm{Mg}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{Mg}\left(\mathrm{s}\right)\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}{\mathrm{E}}^{\circ }=-2.36\mathrm{V}\\ {\mathrm{K}}^{+}+1{\mathrm{e}}^{-}\to \mathrm{K}\left(\mathrm{s}\right)\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}{\mathrm{E}}^{\circ }=-2.93\mathrm{V}\end{array}$

As per electrochemical series, ${\mathrm{Au}}^{3+}$ occupies the top position.