How can the ortho and para isomers formed by electrophilic substitution of arenes can be separated?

Due to the large difference in their melting points

The ortho and para isomers of dichloroarenes have different melting points, so the answer is (3).

The ortho isomer has a higher melting point than the para isomer because the ortho isomer has more steric hindrance. The methyl group in the ortho isomer is closer to the chlorine atoms, which makes it more difficult for the molecules to pack together.

The difference in melting points between the ortho and para isomers is usually small, but it can be large enough to allow them to be separated by fractional distillation.

The densities, solubilities, and boiling points of the ortho and para isomers are similar, so these properties cannot be used to separate them.