Which of the following has sp mixing?

The molecular orbitals of the  ${\mathrm{Cl}}_2,{\mathrm{N}}_2,{\mathrm{B}}_2$ has the same symmetry formed from the combination of 2s and 2p atomic orbitals are close enough in energy to further interact. Hence  ${\mathrm{Cl}}_2,{\mathrm{N}}_2,{\mathrm{B}}_2$ have sp mixing. Hence, the option D is correct.