Explanation

(d)
The isoelectronic species have same number of electrons.
NaCl has $Na^{+}$ and $Cl^{-}$ ions
Electrons in $Na^{+} = 11 - 1 = 10$
Electrons in $Cl^{-} = 17 + 1 = 18$
∴ They are not isoelectronic.
CsF has $Cs^{+}\ and\ F^{-}$ ions
Electrons in $Cs^{+} = 55 - 1 = 54$
Electrons in $F^{-} =$9+1=10
∴ They are not isoelectronic.
NaI has $Na^{+}\ and\ I^{-}$ ions
Electrons in $Na^{+} = 11 - 1 = 10$
Electrons in $I^{-} = 53 + 1 = 54$
∴ These are not isoelectronic.
$K_{2}S$ has $K^{+}$ and $S^{2 -}$ ions
Electrons in $K^{+}\ = 19 - 1 = 18$
Electrons in $S^{2 -} = 16 + 2 = 18$
∴ In $K_{2}S,$ the ions $K^{+}\ and\ S^{2 -}$ are isoelectronic.