Why can you not distinguish between an aldehyde and a ketone?

Well actually you can
Aldehyde contains a H-C=O... the presence of th H aton allows aldehyde to oxidise easily .
However ketone does not have that H atom directly bonded to the C=O (instead it have 2 alkyl group)...

Aldehyde can be oxidised to either a carboxyloc acid under acidic conditions or the salt of a carboxylic acid under alkaline conditions

Aldehyde and ketone xan be distinguish using Tollens reagent or Felhling solution

Tollens reagent...
The aldehyde produces a silvery precipitate (silver mirror).. no chabge is observed with ketone...

Fehlings solution.
Aldehyde: color change from blue to dark red. (Copper (1)oxide)
Ketone..no change...

Note that aldehydes and ketones are carbonyl compound and to detect the presence of carbonyl compound as a whole u use 2,4-dinitrohydrazene.. 2,4-DNPH...
This however cannot tell if the carbonyl is an aldehyde or a ketone!...

Hope it helps

Ithink the answer is c i hope it my friend message me if i’m wrong and i’ll change my answer and fix it for you