Answer: Intermolecular Hydrogen Bonding
Explanation: Ammonia molecule is planar molecule in which 3 hydrogen atoms are attached to the nitrogen atom leaving a pair of lone pair of electrons on the latter.
As we know that the hydrogen bonding is the result of the electrostatic forces of attraction between the hydrogen atom attached with the electronegative atom of one compound with the electronegative atom of the other.
Thus the strongest type of van der waal forces of attraction which can occur in molecules of ammonia is Intermolecular Hydrogen Bonding