Which molecule has the strongest intermolecular force?

Among the 4 options, NH₃ has the strong intermolecular force.

All of the 4 options are small molecules. The intermolecular forces in NH₃ are hydrogen bonds, while the intermolecular forces in either of the 3 other options are van der Waals' forces.

For small molecules, hydrogen bonds are stronger than van der Waals' forces. Therefore, NH₃ has the strongest intermolecular forces among the 4 options.

The answer: C) NH₃

Intermolecular forces.....

In descending order of the force of attraction between molecules.
NH3 ..... Trigonal pyramidal geometry, nonzero dipole moment. Therefore: hydrogen bonding, Keesom forces, Debye forces, London dispersion forces.
H2S ..... Bent molecular geometry, nonzero dipole moment. Therefore, Keesom forces, Debye forces, London dispersion forces.
BF3 ..... Trigonal planar geometry, zero dipole moment. Therefore, only London dispersion forces.
H2 ....... Linear geometry, zero dipole moment. Therefore, only London dispersion forces.

More about intermolecular forces:

1. London dispersion forces .... forces between all molecules. In the past London dispersion forces were characterized as the attraction between temporary dipoles. There may be more to it than that. Dispersion forces are a quantum interaction. All molecules exhibit London dispersion forces, LDF's. The strength of LDF's is proportional to the polarizability of the molecule, which in turn, depends on the number of electrons and the surface area of the molecule. Contrary to what some teachers and even some authors say, in many cases London dispersion forces can be stronger than Keesom or Debye forces and are second only to hydrogen bonding.

2. Keesom forces (dipole-dipole attraction) ... the attraction of one polar molecule for another. Oppositely charged ends of the molecules undergo electrostatic attraction.

3. Debye forces (induced attraction) .... the attraction between a polar molecule and one which is nonpolar, where the polar molecule induces a charge separation in the nonpolar molecule. Debye forces can also exist between two polar molecules, even the same kind of molecule.

4. Hydrogen bonding .... The weakly covalent bonds found between the hydrogen atom of one molecule where it is bonded to N, O or F, and the N, O, or F of an adjacent molecule. The hydrogen atom functions as a bridge, forming a bond between two molecules. Hydrogen bonding is more than an electrostatic attraction between molecules, instead there is evidence of orbital overlap and covalent bond formation.

Note: There are three van der Waals forces (Keesom forces, Debye forces and London dispersion forces). Some textbooks and some teachers' dusty old notes erroneously equate van der Waals forces only with London dispersion forces.