Which of these is the most significant force that attracts polar molecules to each other?

The correct answer is dipole-dipole interactions.

Dipole-dipole forces refer to the forces of attraction among the positive terminal of one polar molecule and the negative terminal of another polar molecule. For example, the bond between hydrogen and chlorine has a dipole-dipole attraction.

A dipole-dipole association refers to the association between the two polar constituents. When they move towards each other, the negative end molecule attracts the positive end of the other molecule.