Given the standard electrode potentials,
$K^{+} / K = -2.93 \ V$,$Ag^{+} / Ag = 0.80 \ V$
$Hg^{2+} / Hg = 0.79 \ V$
$Mg^{2+} / Mg = -2.37 \ V$,$Cr^{3+} / Cr = -0.74 \ V$
Arrange these metals in their increasing order of reducing power.

(A) The reducing power of a metal is inversely proportional to its standard reduction potential.
$A$ lower (more negative) standard electrode potential indicates a stronger reducing agent.
Comparing the given values:
$E^{\circ}_{Ag^+/Ag} = 0.80 \ V$
$E^{\circ}_{Hg^{2+}/Hg} = 0.79 \ V$
$E^{\circ}_{Cr^{3+}/Cr} = -0.74 \ V$
$E^{\circ}_{Mg^{2+}/Mg} = -2.37 \ V$
$E^{\circ}_{K^+/K} = -2.93 \ V$
Arranging these in increasing order of reducing power (from most positive potential to most negative potential):
$Ag < Hg < Cr < Mg < K$.