I figured this out by looking at the periodic table because C is the least electronegative out of the choices.
Another way to do this would be to find out the electronegative difference if you have the Electronegativity table of the Selected Elements.

O-F it'd be 0.5
N-F it'd be 1.0
C-F it'd be 1.4
B-F it'd be 2.0
Cl-F it'd be 0.8

As you can see, B-F is the most electronegative so it is also the most ionic.

View the original article here