In which of the following pairs the two species are not isostructural?

Two species are called isostructural when they possess the same:
 • number of surrounding atoms (same steric number)
 • arrangement of these atoms in space (same molecular geometry / shape).
Hence we compare the VSEPR geometry of each pair.

Case A: $$CO_3^{2-}$$ vs $$NO_3^-$$
Each ion has three $$\sigma$$-bonds to oxygen/nitrogen and no lone pair on the central atom.
Steric number $$=3$$ ⇒ hybridisation $$sp^2$$ ⇒ trigonal-planar geometry.
Therefore the two ions are isostructural.

Case B: $$PCl_4^+$$ vs $$SiCl_4$$
Central $$P$$ in $$PCl_4^+$$: 4 $$\sigma$$-bonds, no lone pair (positive charge has removed one electron).
Central $$Si$$ in $$SiCl_4$$: 4 $$\sigma$$-bonds, no lone pair.
Steric number $$=4$$ for both ⇒ hybridisation $$sp^3$$ ⇒ tetrahedral geometry.
Thus they are isostructural.

Case C: $$PF_5$$ vs $$BrF_5$$
• $$PF_5$$: 5 $$\sigma$$-bonds, 0 lone pairs → steric number $$=5$$ → $$sp^3d$$ hybridisation → trigonal bipyramidal shape.
• $$BrF_5$$: 5 $$\sigma$$-bonds, 1 lone pair → steric number $$=6$$ → $$sp^3d^2$$ electronic geometry (octahedral), but one position occupied by a lone pair, giving a square pyramidal molecular shape.
Since trigonal bipyramidal ≠ square pyramidal, these two molecules are not isostructural.

Case D: $$AlF_6^{3-}$$ vs $$SF_6$$
Both have 6 $$\sigma$$-bonds and no lone pair on the central atom.
Steric number $$=6$$ ⇒ $$sp^3d^2$$ hybridisation ⇒ octahedral geometry.
Hence they are isostructural.

Among the four pairs, only Case C contains species with different shapes. Therefore the pair that is not isostructural is:

Option C which is: $$PF_5$$ and $$BrF_5$$