Be careful! It specifies “complete combustion of fuels”. This makes the whole difference as we will discuss now.
1 – CH4 is not produced in combustion.
2 – CO is produced in INCOMPLETE combustion
3 – CO2 is produced in combustion as all carbon reacts with the oxygen combustion
4 – H2O is produced as the the hydrogen in the hydrocarbons reacts with oxygen during combustion
5 – He is never produced in combustion
6 – NH3 is not produced