Correct answer: by a random, short-lived redistribution of electrons.
van der Waal's forces are the weak intermolecular forces of attraction or repulsions that are seen in all molecules, when they come close to each other. These forces of attraction or repulsion are due to the momentary charges that develop between the electron clouds of one atom and the nuclei of the other atom. Therefore, the van der Waal's forces are due to the instantaneous dipoles that are produced between two atoms close to each other, due to the redistribution of electron clouds.