The end product of one stage fermentation is
All other guides have given ethanol as the answer to this question. Which is incorrect.
One stage fermentation here means fermentation of pyruvate in one stage.
When pyruvate is formed, it can have several fates.
Under aerobic conditions, pyruvate enters the TCA cycle after conversion into acetyl CoA by PDH.
There are two fates for pyruvate under anaerobic metabolism:-
Formic acid
Pyruvate
Lactate
Ethanol