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

Benzaldehyde

The correct answer is Option (3) → Benzaldehyde

Fehling's test detects aldehydes that are not conjugated with an aromatic ring (i.e., aliphatic aldehydes).

• Glucose → reducing sugar (has free aldehyde group) ✅
• Ethanal (CH₃CHO) → aliphatic aldehyde ✅
• Formaldehyde (HCHO) → aliphatic aldehyde ✅
• Benzaldehyde (C₆H₅CHO) → aromatic aldehyde, does not reduce Fehling’s solution ❌