Which main intermolecular force must be overcome to sublime solid NH3?

Sublimation is the phase transition of a substance directly from the solid to the gaseous phase without passing through the liquid phase.

Here are the three forces that exist between two NH3 molecules:

1 - Dispersion forces - these forces exist between ANY two molecules, regardless of anything else.

2 - Dipole-dipole interactions - NH3 is a polar molecule with a permanent separation of charge. Each molecule behaves like a tiny magnet and has + and - poles so 2 NH3 molecules attract each other. Dipole-dipole interactions are stronger than dispersion forces in small molecules.

3 - Hydrogen bonding - H is directly bonded to the extremely electro negative atoms N, O, or F, hence hydrogen atom is nearly stripped off electrons due to the large electro - negativity difference. Hydrogen being a small atom further reduces in size even allowing it to approach other molecules very closely. Here N, O, F gathers more partial negative charge.

Due to the close approach between the molecules and the strong partial charges, hydrogen bonding is generally more important than dipole-dipole interactions.

So the main intermolecular force must be hydrogen bonding.