Fehling's test identifies reducing agents via red Cu2O precipitate Steps:

• Fehling's reagent oxidizes aldehydes and similar reducers, yielding red Cu2O precipitate.
• HCO2H reduces Fehling's like aldehydes due to its oxidizable formyl group.
• Evaluate second compound in each pair for matching red precipitate reaction.
• Only aldehydes like CH3CH2CHO match HCO2H's positive response.

Why D is correct:

• Both HCO2H and CH3CH2CHO (propanal, RCHO where R=CH3CH2) reduce Fehling's [Cu^{2+}] to Cu2O red precipitate, per the standard aldehyde oxidation equation: RCHO + 2Cu^{2+} + 5OH^- → RCO2^- + Cu2O + 3H2O.

Why the others are wrong:

• A: CH3CO2H lacks reducing power as a typical carboxylic acid, giving no precipitate.
• B: CH3CHO reduces Fehling's but pairs with the basic aldehyde example, not matching the question's focus on equivalent oxidation behavior.
• C: CH3CH2OH is a primary alcohol that does not reduce Fehling's, giving no precipitate.

Final answer: D