a
From ideal gas equation, $P V=n R T$
Here, assume the mass of helium in the mixture is $x$, then the mass of hydrogen is $(5-x)$
Applying ideal gas law,
$2 \times 10^{5} \times 0.02=\left(\frac{5-x}{2}+\frac{x}{4}\right) \times 8.314 \times 293$
$x=3.43$ and $5-x=1.57$
So, the ratio of hydrogen to helium is $1: 2 .$