1. Identify the isomers

Dichlorobenzene has three positional isomers:

• $o$-dichlorobenzene $(1,2-)$
• $m$-dichlorobenzene $(1,3-)$
• $p$-dichlorobenzene $(1,4-)$

We need the correct order of their melting points.

2. Key principle: melting point depends strongly on crystal packing

For isomeric aromatic compounds, melting point is not determined only by molecular mass. It depends a lot on how well molecules can pack in the crystal lattice.

• More symmetrical molecules pack better.
• Better packing gives stronger intermolecular attractions in the solid state.
• Hence, more symmetrical isomers generally have higher melting points.

3. Compare symmetry of the three isomers

• $p$-dichlorobenzene is the most symmetrical.
• $m$-dichlorobenzene is less symmetrical.
• $o$-dichlorobenzene is also less symmetrical and its adjacent chlorine atoms cause poorer packing.

Therefore, the highest melting point should be for the para isomer.

4. Known relative order

The standard order of melting points is:

$p\text{-dichlorobenzene} > o\text{-dichlorobenzene} > m\text{-dichlorobenzene}$

Approximate values support this:

• $p$-dichlorobenzene: about $53^\circ C$
• $o$-dichlorobenzene: about $-17^\circ C$
• $m$-dichlorobenzene: about $-24^\circ C$

So,

$m.p.:\quad p > o > m$

5. Match with stored answer

Since the option texts are not visible, I can only derive the correct order as:

$p\text{-dichlorobenzene} > o\text{-dichlorobenzene} > m\text{-dichlorobenzene}$

If option A corresponds to this order, then the stored answer is correct.