Isoelectronic species are elements or ions that have the same or an equal number of electrons.
A. $) Cl _{2} O =2 \times 17+8=42$ electrons
$ICl _{2}^{-}=53+2 \times 17+1=88$ electrons
B. ) $Cl _{2}^{-}=2 \times 17+1=35$ electrons
$ClO _{2}=17+2 \times 8=33$ electrons
C. $) I F_{2}^{+}=53+2 \times 8-1=68$ electrons
$I _{3}{ }^{-}=53 \times 3+1=160$ electrons
D. ) $ClF _{2}^{+}=17+2 \times 9-1=34$ electrons
$ClO _{2}^{-}=17+2 \times+1=34$ electrons
$ClO _{2}^{-}$and $ClF _{2}^{+}$contain 34 electrons.
Hence option D contains the same number of electrons. Therefore, they are isoelectronic.