The reaction of ${\mathrm{P}}_4$ with $\mathrm{X}$ leads selectively to ${\mathrm{P}}_4{\mathrm{O}}_6$. Then $\mathrm{X}$ is :

Phosphorus is heated with a little amount of oxygen and nitrogen to produce phosphorus trioxide $\left({\mathrm{P}}_4{\mathrm{O}}_6\right)$ 

${\mathrm{P}}_4+3{\mathrm{O}}_2\stackrel{{\mathrm{O}}_2+{\mathrm{N}}_2}{\to }{\mathrm{P}}_4{\mathrm{O}}_6$