Acetaldehyde and acetone can be identified by?

In both carbonyl group is common, Aldehydes give silver mirror (black precipitate) with Tollens’ reagent, but not ketones. 

Aldehydes restore the pink color of Schiff’s base, but not ketones. 

Luca test is used to differentiate alcohol.2,4 DNP test is used to identify whether the carbonyl group is present or not in a given compound. 

Hence the correct option is: (B) 2,4-DNP test