O-H bond
Explanation:
Let us work out the electronegativity difference between the elements in each bond in order to decide which of them is most polar.
For the C-O bond
2.55 - 2.2 =0.35
For the F-F bond
3.98 - 3.98 = 0
For the O-H bond
3.44 - 2.2 = 1.24
For the N-H bond
3.04 - 2.2 = 0.84
The O-H bond has the highest electronegativity difference, hence it is he most polar bond.