The bond C-C has a higher energy than C-N because the bond C-N has 1 more unpaired electron than C-C, and unpaired electrons produce repulsion which alters the energy reducing it.
The bond C-F is the stongest because the F atom is very small, also has a very strong electronegativity and has no unpaired electrons so that makes it more stable.
The unpaired electrons in N atom, are those up and down the N atom.