Which substance is NOT produced during the combustion of alkanes and why?(a) CO2(b) CO(c) C(d) H2 The answer is apparently D but i don't understand why it can't be the other 3

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The answer is (d) H2.

I will explain how you can get first three.

CO2 - If an alkane is undergoing complete combustion(i.e. in excess O2), the products will be CO2 and H2O.

On the other hand if iy doesnt undergo complete combustion (with less amount of O2), it will produce CO2 + CO and H2O

If oxygen levels drop further down, you can sometimes get soots which are obviously unburnt C particles.
 In this case you will get, CO2+CO+C+H2O.

But you will never be able to get H2 as a product of combustion, even though you can get H2, if an alkane is subjected to pyrolysis.

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