Which of the following sets of ions represents a collection of isoelectronic species?

$\begin{array}{l}a.N^{3-}=7+3=10e^{-},O^{-}\to 8+2=10e\\ P^{-}=9+1=10e^{-},S^{--}\to 16+2=18e^{-}\end{array}$

                                                   (not iso electronic)

    $\begin{array}{l}b.L{i}^{+}=3+1=4e^{-}.N{a}^{+}=11-1=10e^{-},\\ M{g}^{++}=12-2=10e^{-}\\ C{a}^{++}=20-2=18e^{-}\left(notisoelectronic\right)\end{array}$

      $\begin{array}{l}c.B{a}^{++}56-2=54e^{-},S{r}^{++}38-2=36e^{-}\\ K^{+}=19-1=18e^{-},C{a}^{++}=20-2=18e^{-}\left(notisoelectronic\right)\end{array}$