If equal masses of different compounds undergo complete combustion, the compound that yields the greatest mass of CO2 will be the one that contains the most carbon. This is because CO2 is produced when carbon in a compound reacts with oxygen during combustion.
Here are the masses of CO2 that would be produced by equal masses of each of the compounds you listed if they underwent complete combustion:
Methane (CH4): One molecule of methane contains one atom of carbon and four atoms of hydrogen. When it undergoes complete combustion, it reacts with two molecules of oxygen (O2) to produce one molecule of CO2 and two molecules of water (H2O). Therefore, the mass of CO2 produced by one molecule of methane is 44 g/mol (the molar mass of CO2 is 44 g/mol).
Ethanol (C2H5OH): One molecule of ethanol contains two atoms of carbon, six atoms of hydrogen, and one atom of oxygen. When it undergoes complete combustion, it reacts with three molecules of oxygen to produce two molecules of CO2 and three molecules of water. Therefore, the mass of CO2 produced by one molecule of ethanol is 88 g/mol (the molar mass of CO2 is 44 g/mol).
Octane (C8H18): One molecule of octane contains eight atoms of carbon and 18 atoms of hydrogen. When it undergoes complete combustion, it reacts with 25 molecules of oxygen to produce 16 molecules of CO2 and 18 molecules of water. Therefore, the mass of CO2 produced by one molecule of octane is 704 g/mol (the molar mass of CO2 is 44 g/mol).
Based on these calculations, octane would yield the greatest mass of CO2 when it undergoes complete combustion. Methane and ethanol would yield less CO2, since they contain less carbon.