At room temperature, ${\mathrm{NH}}_3$ and HX are passed in a ‘1 m’ long tube from two opposite ends at the same pressure. The formation of ${\mathrm{NH}}_4$(s) will be closest from the injection of HX end when HX is

As per Graham’s law,

Diffusion $\mathrm{\alpha } \sqrt{\frac{1}{\mathrm{molecular} \mathrm{mass}}}$

As the molar mass of HI is high than the molar mass of ${\mathrm{NH}}_3$ , the formation of ${\mathrm{NH}}_4$ X (s) will be closest from the injection of HX end when HX is HI.