The correct order of $\mathrm{N}-\mathrm{O}$ bond length in the given compounds is :

As the electronegativity of  $\mathrm{F},\mathrm{Cl},\mathrm{Br}$ decreases in the order $\mathrm{F}>\mathrm{Cl}>\mathrm{Br},s\text{-character in }\mathrm{O}=\mathrm{N}$ bond decreases and decreases and $\mathrm{N}\Rightarrow \mathrm{O}$ bond length increases.

Hence, the correct answer is $\mathrm{NOF}>\mathrm{NOCl}>\mathrm{NOBr}$.