Ammonia gas diffuses twice as fast as does gas X. The gas 'X' is

According to Graham's law 

Rate of diffusion is inversely proportional to the square root of the molecular mass

Therefore

      $\frac{{\mathrm{r}}_1}{{\mathrm{r}}_2}=\sqrt{\frac{{\mathrm{M}}_2}{{\mathrm{M}}_1}}$

$\frac{2}{1}=\sqrt{\frac{{\mathrm{M}}_2}{17}}$

${\mathrm{M}}_2=68$

So that the molar mass of ${\mathrm{C}}_5{\mathrm{H}}_8$ is nearly equal to $5\times 12+8\times 1=68$

So the correct option is C