The ratio of rate of diffusion of helium with respect to methane under similar conditions of constant temperature 

and pressure is :

Molecular weight of

       ${\mathrm{CH}}_4=12+(4\times 1)=16\mathrm{amu}$

At wt. of $\mathrm{He}=\text{ Mol. wt. of }\mathrm{He}=4\mathrm{amu}.$

We know that rate of diffusion (r) of gases is inversely

 proportional to the square root of their molecular weights. 

Hence  $\begin{array}{r}\frac{r_{\mathrm{He}}}{r_{{\mathrm{CH}}_4}}={\left(\frac{M_{{\mathrm{CH}}_4}}{M_{\mathrm{He}}}\right)}^{1/2}={\left(\frac{16}{4}\right)}^{1/2}\\ =(4{)}^{1/2}=2.\end{array}$

So, the correct answer is (a).