The compound which will not respond to Fehling's test is

Benzaldehyde

The correct answer is Option (3) → Benzaldehyde

Fehling's test is used to detect aliphatic aldehydes and reducing sugars. In this test, aldehydes reduce Cu²⁺ ions in Fehling's solution to Cu₂O, forming a brick-red precipitate. The reaction occurs easily with aliphatic aldehydes, but aromatic aldehydes generally do not give this test because they are less reactive toward oxidation under these conditions.

Glucose

Glucose is a reducing sugar. In aqueous solution, it can open to form an aldehyde group, which reduces Fehling's reagent. Thus, glucose gives a positive Fehling's test.

Ethanal

Ethanal is an aliphatic aldehyde, which readily reduces Fehling's solution to produce a brick red precipitate of Cu₂O. Thus, it responds to Fehling's test.

Benzaldehyde

Benzaldehyde is an aromatic aldehyde. Aromatic aldehydes generally do not reduce Fehling's solution because the benzene ring stabilizes the carbonyl group, making oxidation difficult. Thus, benzaldehyde does not respond to Fehling's test.

Formaldehyde

Formaldehyde is an aliphatic aldehyde and easily reduces Fehling's reagent. Thus, it gives a positive Fehling's test.