Missing information:
Fossil fuel + oxygen gas  -> _________ + water + heat
Answer:
Carbon dioxide (CO2)
Explanation:
Burning of fossil fuel in the presence of oxygen results into production of carbon dioxide (CO2). This is because of the reason that, in the presence of oxygen, carbon is oxidized to form CO2. The hydrogen in fossil fuel will also reacts with oxygen to form water. The overall processes produce intensive heat/energy. This process is also called as combustion.
