Which of the following combination contains only isoelectronic series?

The combination contains only isoelectronic series is ${\mathrm{P}}^{3-}, {\mathrm{S}}^{2-},{ \mathrm{Cl}}^{-}, \mathrm{Ar}$.

Isoelectronic species are the species having same number of electrons.

here ${\mathrm{P}}^{3-}, {\mathrm{S}}^{2-},{ \mathrm{Cl}}^{-}, \mathrm{Ar}$ Isoelectronic species having 18, knot electron each.

Hence, the option C is correct.