What is the most polar?

c-n
c-o
c-c
c-cl
c-f

C-C is a nonpolar covalent bond since the two atoms are the same and both non-metals. we are left with the polar covalent bonds and to determine the most polar one, we identify the electronegative difference of the two elements. In this case, the largest  electronegative difference is of C-F. 

It’s the energy source of stars