$\mathrm{In} \mathrm{the} \mathrm{process} 2{\mathrm{PCl}}_5\rightleftharpoons {\mathrm{PCl}}_4^{+}+{\mathrm{PCl}}_6^{-}$  the change in hybridisation is

${\mathrm{sp}}^3\mathrm{d} \mathrm{to} {\mathrm{sp}}^2 \mathrm{and} {\mathrm{sp}}^3{\mathrm{d}}^2$

${\mathrm{sp}}^3\mathrm{d} \mathrm{to} {\mathrm{sp}}^2 \mathrm{and} {\mathrm{sp}}^3$

${\mathrm{sp}}^3{\mathrm{d}}^2 \mathrm{to} {\mathrm{sp}}^3 \mathrm{and} {\mathrm{sp}}^3\mathrm{d}$

${\mathrm{sp}}^3\mathrm{d} \mathrm{to} {\mathrm{sp}}^3 \mathrm{and} {\mathrm{sp}}^3{\mathrm{d}}^2$