s-p mixing is not possible in among the following:

${\mathrm{C}}_2,{\mathrm{B}}_2,\mathrm{NO},\mathrm{CO},{\mathrm{NO}}^{+},{\mathrm{C}}_2^{2+},{\mathrm{O}}_2^{2+},{\mathrm{O}}_2^{+},{\mathrm{N}}_2,{\mathrm{N}}_2^{+},{\mathrm{N}}_2^{-}$

The assumption that the s and p orbitals mix on the same atom also appears to be in conflict with this, since there is no s-p mixing in ${\mathrm{O}}_2$.

s- p mixing is not possible in ${\mathrm{O}}_2^{2+},{\mathrm{O}}_2^{+}$

Thus, the correct answer is 2.