Final answer:
Coal releases much more CO2 than natural gas due to its lower energy density and higher mass ratio.
Step-by-step explanation:
Coal produces more CO2 per gram of fuel compared to natural gas due to its lower energy density and higher mass ratio. While all forms of fossil fuels produce a mass ratio of approximately 3 units of CO2 for every unit of fuel, coal has a slightly higher mass ratio and is more than twice as carbon-intense as natural gas. Therefore, coal releases much more CO2 than natural gas.