The gas that can diffuse slowly is

As per Graham’s law of diffusion, rate of diffusion of gas is inversely proportional to square root of its molecular mass.

$\text{Rate,\hspace{0.17em}r}\propto \frac{1}{\sqrt{\text{M}}}$

Here ‘M’ is molecular mass of a gas.

${\text{For NH}}_3\text{\hspace{0.17em}\hspace{0.17em}gas},\text{\hspace{0.17em}\hspace{0.17em}M.W\hspace{0.17em}=\hspace{0.17em}}17$

For $C{H}_4$ gas, M. W = 16

For $O_2$ gas, M.W = 32;  For $H_2$ gas, M.W = 2

From the above formula it is known that the gas which has least molar mass that can diffuse easily than other gases and the gas which has high molar mass that can diffuse slowly.

Therefore, among given all gases hydrogen gas, $H_2$ diffuse more easily and  $O_2$ gas diffuses slowly.