A polar bond is formed with atoms having different electronegativities. The bonding electrons are attracted more towards the atom with greater electronegativity resulting in unequal sharing of electrons. Therefore the molecule develop partial charges and becomes polar. Polar molecules have dipole moment that is the partial charge on molecules due to differences in electronegativity between atoms.
A non-polar bond is formed with atoms having the same electronegativity, hence the bonded pair of electron is shared equally between atoms. Non-polar molecules have no moment.
Note that: symmetrical molecules having polar bonds are non-polar because the dipoles of the bond exert equal and opposite effect. Hence the dipoles cancel the charges.
Example: HCl
In HCl, Cl is more electronegative therefore Cl atom pulls the electron pair of the covalent bond towards itself and develops a partial negative charge. Consequently H develops a partial positive charge. This therfore leads to the formation of a dipole.
The molecule BH3 is trigonal planar, with B in the center and H in the three vertices. Ther are no free electrons. All the valence electrons are paired in and forming bonds.
There are four kind of intermolecular attractions: ionic, hydrogen bonds, polar and dispersion forces.
B and H have very similar electronegativities, Boron's electronegativity is 2.0 and Hydrogen's electronegativity is 2.0.
The basis of ionic compounds are ions and the basis of polar compounds are dipoles.
The very similar electronegativities means that B and H will not form either ions or dipoles. So, that discards the possibility of finding ionic or polar interactions.
Regarding, hydrogen bonds, that only happens when hydrogen bonds to O, N or F atoms. This is not the case, so you are sure that there are not hydrogen bonds.
When this is the case, the only intermolecular force is dispersion interaction, which present in all molecules.
Then, the answer is dispersion interaction.
There doesn't seem to be a clear answer, it is placed in the center when it's substances is on the corners it doesn't give anything else