Acetaldehyde and benzaldehyde can be best distinguished by:

The correct answer is option 4. Fehling's test.

The best reagent to distinguish between acetaldehyde and benzaldehyde is Fehling's test.

2,4-DNP test: This test detects the presence of carbonyl groups (C=O) and can identify both acetaldehyde and benzaldehyde.

Tollens' test: Similar to Fehling's test, Tollens' reagent also reacts with aldehydes like acetaldehyde and benzaldehyde. It wouldn't differentiate between them.

Sodium bicarbonate test: This test reacts with acidic compounds. While some aldehydes can exhibit weak acidity, it's not a reliable test to distinguish specifically between acetaldehyde and benzaldehyde.

Fehling's test: This test specifically relies on the presence of alpha-hydrogens (hydrogens on the carbon next to the carbonyl group).

Acetaldehyde \((CH_3CHO)\) has alpha-hydrogens, while benzaldehyde \((C_6H_5CHO)\) does not have any \(\alpha \)-hydrogens due to the presence of a benzene ring directly attached to the carbonyl group.

In Fehling's test, the \(\alpha \)-hydrogens in acetaldehyde are essential for the reaction. These hydrogens participate in the formation of an enolate intermediate, which then reacts with cupric ions \((Cu^{2+})\) in Fehling's solution, leading to a characteristic red precipitate of cuprous oxide \((Cu_2O)\). Benzaldehyde, lacking alpha-hydrogens, wouldn't undergo this reaction and wouldn't give a positive test result.

Therefore, Fehling's test allows you to distinguish between acetaldehyde and benzaldehyde based on the presence or absence of \(\alpha \)-hydrogens.