Reduction potential of ions are given below:

${\mathrm{ClO}}_{4 }^{-} {\mathrm{IO}}_4^{-} {\mathrm{BrO}}_4^{-}$

$\mathrm{E}°=1.19 \mathrm{V} ; \mathrm{E}°=1.65 \mathrm{V}; \mathrm{E}°=1.74 \mathrm{V}$

The correct order of their oxidising power is:

The reduction potential of the substance is the ability of the substance to be reduced. So as the reduction potential increases, the reducing ability of the substance increases. It means the oxidising power of the substance (The ability of the substance to make the other substance to lose the electrons increases, which nothing but oxidising power) increases.

So the decreasing order of oxidising power is ${\mathrm{BrO}}_4^{-}>{\mathrm{IO}}_4^{-}>{\mathrm{ClO}}_4^{-}$.

Hence option B is correct.