The ion that is isoelectronic with $$CO$$  is

Solution :
Isoelectronic species are having same number of electrons.
Number of electrons in $$CO = 6 + 8 = 14$$
Number of electrons in $$O_2^ - = 16 + 1 = 17$$
Number of electrons in $$N_2^ + = 14 - 1 = 13$$
Number of electrons in $$O_2^ + = 16 - 1 = 15$$
Number of electrons in $$C{N^ - } = 6 + 7 + 1 = 14$$
Hence, $$CO$$  isoelectronic with $$C{N^ - }$$  ion.