Which of the following has the highest melting point?

The correct answer is Option (4) → p-Dichlorobenzene

The melting point depends on how well the molecules pack in the crystal lattice. The para-isomer is the most symmetrical, so it fits into the lattice best and has the highest melting point.

Why the other options are incorrect

Option (1) Chlorobenzene — Incorrect. It has only one chlorine atom: a smaller molecular mass and a less symmetrical shape than p-dichlorobenzene.

Option (2) o-Dichlorobenzene — Incorrect. The two chlorine atoms on adjacent carbons make it less symmetrical, so it packs less well than the para-isomer.

Option (3) m-Dichlorobenzene — Incorrect. It is also less symmetrical than the para-isomer and packs less well.

NCERT (2026-27 edition):
6.6 Physical Properties – “Boiling points of isomeric dihalobenzenes are very nearly the same. However, the para-isomers are high melting as compared to their ortho- and meta-isomers. It is due to symmetry of para-isomers that fits in crystal lattice better as compared to ortho- and meta-isomers.”