Firstly, ethanol was excluded, and 2,4-dinitrophenylhydrazine and carbonyl were precipitated to identify ethanol.

Then the aldehyde and ketone were identified by Toulon reagent (silver mirror reaction), and acetone did not produce white precipitate.

Then, aromatic aldehyde and fatty aldehyde were identified by film-forming reagent, and benzaldehyde was identified because aromatic aldehyde did not produce brick-red precipitate.

Finally, the ketone structure of acetaldehyde was identified by iodoform reaction.

The last thing left is formaldehyde.

The answer upstairs does not identify ethanol.