Given:
$Co^{3+} + e^- \longrightarrow Co^{2+}; E^o = 1.81 \ V$
$Pb^{4+} + 2e^- \longrightarrow Pb^{2+}; E^o = + 1.67 \ V$
$Ce^{4+} + e^- \longrightarrow Ce^{3+}; E^o = + 1.61 \ V$
$Bi^{3+} + 3e^- \longrightarrow Bi; E^o = + 0.20 \ V$
Oxidizing power of the species will increase in the order:

• A
  $Ce^{4+} < Pb^{4+} < Bi^{3+} < Co^{3+}$
• B
  $Co^{3+} < Pb^{4+} < Ce^{4+} < Bi^{3+}$
• C
  $Bi^{3+} < Ce^{4+} < Pb^{4+} < Co^{3+}$
• D
  $Co^{3+} < Ce^{4+} < Bi^{3+} < Pb^{4+}$