Species having same number of electrons are isoelectronic. On calculating the number of electrons in each species given here, we get. CN−(6+7+l=14);N2(7+7=14) O22−(8+8+2=18);C22(6+6+2=14) O2−(8+8+1=17);NO+(7+8−1=14) CO(6+8=14);NO(7+8=15) From the above calculation we find that all the species listed in choice (b) have 14 electrons each so it is the correct answer.