The correct order of melting point of dichlorobenzenes is

The melting point of a compound depends heavily on its crystal lattice packing efficiency and molecular symmetry, rather than just its dipole moment.
para-Dichlorobenzene (Highest Melting Point): The para isomer is highly symmetrical. This symmetry allows the molecules to pack very closely and tightly into the crystal lattice.
ortho-Dichlorobenzene (Intermediate Melting Point): The ortho isomer has a high dipole moment due to the close proximity of the two chlorine atoms, leading to relatively strong dipole-dipole interactions, which gives it a higher melting point.
meta-Dichlorobenzene (Lowest Melting Point): The meta isomer lacks the perfect symmetry of the para form and doesn't possess a dipole moment as strong as the ortho form. This results in weaker packing efficiency and weaker intermolecular forces.

Therefore, Option D is correct.