Arrange the following compounds in increasing order of length of their $\mathrm{C}-\mathrm{N}$ bond.

 

The bond order of $\mathrm{C}-\mathrm{N}$ bond is 2 in III, shortest, I in II, longest bond. There is greater extent of delocalization of nitrogen lone pair in IV because it brings negative charge on oxygen than in I where negative charge moves on carbon. Hence, $\mathrm{C}-\mathrm{N}$ bond acquire greater partial double bond
character in IV than in I. This results in greater shortening of $\mathrm{C}-\mathrm{N}$ bond length in IV than in I, although I has more resonating structures.