How would you arrange the following bonds in order of increasing ionic character: #C-F#, #O-F#, #H-F#, and #Na-F#?
Lowest ionic character O-F, then C-F, then H-F, and finally greatest ironic character in Na-F.
As a general guide, ionic character (as opposed to covalent) increases in direct proportion with the difference in electronegativity between the two bonded atoms.
If the difference in electronegativity is very large, you will get a more ionic bond (e.g. a metal and a halogen) or if its comparatively small (e.g. sulphur and oxygen) you will get a more covalent bond.
F - most electronegative
Na - least electronegative
Therefore the smallest electronegativity difference (least ionic character) will be found between O-F, then C-F, then H-F, and finally the greatest electronegativity difference (most ionic character) will be between Na - F.
Another way to do it is to look up values of Pauling Electronegativity for each element - then you can work out actual values of electronegativity difference for each pair, which makes ranking them even easier.
Pauling values are:
F - 3.98
O - 3.44
C - 2.55
H - 2.20
O - F - difference 0.54
C - F - difference 1.43
H - F - difference 1.78
Na - F - difference 3.05