Solution

In case of dichlorobenzenes, para isomer is more symmetrical than ortho and meta isomers.
Hence, in the crystal lattice, para isomer fits more closely than ortho and meta isomers.
Due to this, more energy is required to break the crystal lattice of para isomer.
Hence, p-Dichlorobenzene has higher melting point than those of o- and m-isomers.