Assuming that the gas acts like an ideal gas so that the molar volume is Vm = 22.4 L / moles, we can calculate for the number of moles.
number of moles = 6 L / (22.4 L / mol) = 0.2679 moles
So the molar mass of this gas is:
molar mass = 8 g / 0.2679 moles = 29.9 g/mol
The value is around 32 g/mol which is the molar mass of oxygen. Therefore the answer is:
O2