The $\ce{BF3}$ molecule is nonpolar, whereas the $\ce{NF3}$ molecule is polar. Which of the following statements accounts for the difference in polarity of the two molecules?

Going through all the answer choices:

In both molecules, only single bonds exist.

Both bonds are polar, since the atoms between the bonds are different.

Both molecules are molecular compounds, since they are formed from covalent, not ionic, bonds.

There is a difference in molecular geometry. We would need to draw the lewis structures to confirm this:

Since $\ce{NF3}$ has four regions of electrons (with the lone-pair electrons taking more space), it will have a trigonal pyramidal molecular geometry. The electrons are therefore not distributed evenly in space, and the molecule will have a dipole moment.

$\ce{BF3}$ is trigonal planar and symmetrical, causing dipole moments to effectively cancel out.