Which of the following BEST distinguishes an aldehyde from a ketone in the lab?

1. The key chemical difference: ALDEHYDES are easily oxidised to carboxylic acids; KETONES are not (under mild conditions). 2. Tollens' reagent ($\mathrm{[Ag(NH_3)_2]^+}$ in mildly basic solution) is reduced by aldehydes to metallic silver, producing the characteristic SILVER MIRROR on the test tube wall. Ketones give NO mirror. 3. Fehling's solution (Cu$^{2+}$ in basic tartrate) is similarly reduced by aldehydes to a red-brown Cu$_2$O precipitate. Ketones don't react (except for $\alpha$-hydroxy ketones). 4. Options A, B, C: bisulphite addition, hydrazone formation, and HCN addition all react with BOTH aldehydes and most simple ketones — they identify the carbonyl group but don't distinguish the two classes. 5. Tollens' + Fehling's together are the textbook PMT/NEET test for 'is it an aldehyde?' _Source: NCERT Class 12 Chemistry Part 2, Ch 8, §8.4 (Chemical Reactions — oxidation; Tollens' and Fehling's tests), p. 11–13._