Which of the following are isoelectronic species, i.e., those having the same number of electrons?

Na → 1s²,2s²2p⁶,3s¹
Na⁺ → 1s²,2s²2p⁶
Similarly for Mg²⁺ → 1s²,2s²2p⁶
Na⁺, Mg²⁺ are isoelectronic species. They contain 10 electrons each.
S²⁻ → 1s²,2s²2p⁶,3s²3p⁶
Ar → 1s²,2s²2p⁶,3s²3p⁶
K⁺ → 1s²,2s²2p⁶,3s²3p⁶
Ca²⁺ → 1s²,2s²2p⁶,3s²3p⁶
Ar, S²⁻, Ca²⁺ and K⁺ are isoelectronic species. They contain 18 electrons each.