Which statement best describes london dispersion forces?

It is also called as van der  Waal's force.

These forces always operate in any substance. The force arisen from induced dipole and the interaction is weaker than the dipole-dipole interaction. All in all, the heavier the molecule, the stronger the van der Waal's force of interaction. For example, the boiling points of inert gases increase as their atomic masses increases due to stronger London dispersion interactions.