What is the strongest type of van der waals force that exists between molecules of ammonia, nh3?

 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

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.