the ion that is isoelectronic to CO is

a) O2-

b) N2+

c) CN-

d) O2+

The correct answer is (c) CN^-

Isoelectronic species are those species which have the same number of electrons. CO in total has 14 electrons (6 from carbon and 8 from oxygen). Out of the given ions CN^- is the ion which has 14 electrons (6 from carbon, 7 from nitrogen and 1 from the negative charge). Thus CN^- ion is isoelectronic with CO.