Which of the following gases will diffuse at a slowest rate at the same temperature ?

According to Graham law of diffusion,

$\text{Rate, r}\propto \frac{1}{\sqrt{\text{M}}};(\text{Here\hspace{0.17em}}\text{'}\text{M' is molar mass})$

For Hydrogen $\left({\text{H}}_{\text{2}}\right)$ gas, M.W = 2;  For Helium (He), M.W = 4

For Nitrogen ${\text{(N}}_2)$ = 28;  For HCl gas, M.W = 36.5

In the above all gases, HCl has more molecular weight.

Therefore, HCl has slowest rate of diffusion.