Positive Fehling solution test is given by

The correct answer is 3.

Fehling's Solution Test Concept:

Fehling's test is a chemical test used to distinguish between aliphatic aldehydes and aromatic aldehydes or ketones.

1. Aliphatic aldehydes (such as CH₃CHO, HCHO, cyclohexanecarbaldehyde) reduce copper(II) ions in Fehling's solution to give a red precipitate of cuprous oxide (Cu₂O). Thus, they give a positive Fehling's test.

2. Aromatic aldehydes (such as benzaldehyde or p-nitrobenzaldehyde) do not give a positive Fehling's test because the aldehyde group (-CHO) is attached directly to an aromatic benzene ring, making oxidation under these conditions difficult.

Analyzing the given compounds in the image:

Let's evaluate each of the 5 compounds shown from left to right:

1. Benzaldehyde (C₆H₅CHO): This is an aromatic aldehyde. Therefore, it gives a negative Fehling's test.

2. Acetaldehyde (CH₃CHO): This is an aliphatic aldehyde. Therefore, it gives a positive Fehling's test.

3. Formaldehyde (HCHO / H-C(=O)-H): This is an aliphatic aldehyde. Therefore, it gives a positive Fehling's test.

4. p-Nitrobenzaldehyde (O₂N-C₆H₄-CHO): This is an aromatic aldehyde. Therefore, it gives a negative Fehling's test.

5. Cyclohexanecarbaldehyde (C₆H₁₁CHO): This is an aliphatic (alicyclic) aldehyde since the -CHO group is attached to a non-aromatic cyclohexane ring. Therefore, it gives a positive Fehling's test.

Conclusion:

Out of the 5 given compounds, exactly 3 compounds (CH₃CHO, HCHO, and cyclohexanecarbaldehyde) give a positive Fehling's solution test.