The correct option is C 1 atm
Use dalton's law of partial pressures.
Take the equal mass to be 'm'
Total moles of gases added: m/4 +m/40
Since no reaction is happening, number of moles are proportional to the partial pressures.
Hence, the partial pressures are in the ratio 10:1, and the partial pressure of the lighter gas (4g/mol) would be 1 atm.