The correct option is C 80%
For gases, mole and volume ratios are the same at a fixed temperature and pressure.
Since, the mixture contains 50% helium and 50% methane by volume so, it will have 1:1 mole ratio. Let the mixture contains 1 mole of He and 1 mole of CH4.
Mass of 1 mole of CH4=16 g
Mass of 1 mole of He=4 g
Total mass of mixture =20 g
Percentage by mass of CH4
=mass of CH4Total mass×100=1620×100=80
∴ Percentage of CH4 present in the mixture by mass =80%