Principle used

Oxidizing power of a species depends on its tendency to get reduced.

The greater the standard reduction potential $E^\circ$, the greater is the tendency to gain electrons, and hence the stronger oxidizing agent.

Given standard reduction potentials

$Co^{3+} + e^- \to Co^{2+}, \quad E^\circ = +1.81\,\text{V}$ $Pb^{4+} + 2e^- \to Pb^{2+}, \quad E^\circ = +1.67\,\text{V}$ $Ce^{4+} + e^- \to Ce^{3+}, \quad E^\circ = +1.61\,\text{V}$ $Bi^{3+} + 3e^- \to Bi, \quad E^\circ = +0.20\,\text{V}$

Arrange by increasing oxidizing power

Since oxidizing power increases with increasing $E^\circ$:

$0.20 < 1.61 < 1.67 < 1.81$

Therefore,

$Bi^{3+} < Ce^{4+} < Pb^{4+} < Co^{3+}$

Match with options

This corresponds to Option D.

Conclusion

The increasing order of oxidizing power is:

$\boxed{Bi^{3+} < Ce^{4+} < Pb^{4+} < Co^{3+}}$