Urotropine is an important organic compound formed from HCHO and${\mathrm{NH}}_3$ • It's molecular formula is ${\left({\mathrm{CH}}_2\right)}_6{\mathrm{N}}_4$. The number of methylene bridges present per nitrogen molecule are :

The structure of ${\left({\mathrm{CH}}_2\right)}_6{\mathrm{N}}_4$ can be given as,

Here, in the structure it is clearly seen that the methylene bridges present per nitrogen molecule are 3.

Thus, the correct answer is 3.