The d-orbital used in${\mathrm{sp}}^3\mathrm{d}$ hybridization forming square pyramidal geometry is :

The d-orbital used in${\mathrm{sp}}^3\mathrm{d}$ hybridization forming square pyramidal geometry is $d_{x^2-y^2}$.

The order of filling the orbital is as

$s+p_x+p_y+p_z+d_{x^2-y^2}$

It is obtained that the d orbital is in the last. Hence, The d-orbital used in${\mathrm{sp}}^3\mathrm{d}$ hybridization forming square pyramidal geometry is $d_{x^2-y^2}$.

Hence, the option B is correct.