Samples of benzaldehyde and acetaldehyde can be distinguished by using

Mixture of Fehling solution A + B

The correct answer is 3. Mixture of Fehling solution A + B.

Although acetaldehyde and benzaldehyde are both aldehydes yet aliphatic aldehydes reduceFehling's solution but aromatic aldehydes do not. Thus, acetaldehyde gives a red precipitate of \(Cu_2O\) with Fehling's solution but benzaldehyde does not.