α,β-Unsaturated aldehyde identification Steps:

• Fehling's solution detects reducing agents like aldehydes, producing red-brown Cu₂O precipitate.
• Cis-trans isomerism requires a carbon-carbon double bond with different substituents on each carbon.
• Compound P must be an aldehyde (for Fehling's test) with an internal C=C bond (for geometric isomerism).
• Thus, P is an α,β-unsaturated aldehyde like but-2-enal, which has E/Z isomers and reduces Fehling's.

Why D is correct:

• D (but-2-enal, CH₃CH=CHCHO) has a C=C bond enabling cis-trans isomers and an aldehyde group that reduces Cu²⁺ to Cu₂O.

Why the others are wrong:

• A (ethanal, CH₃CHO): Saturated aldehyde; no C=C for cis-trans isomerism.
• B (propanone, CH₃COCH₃): Ketone; does not reduce Fehling's and lacks C=C.
• C (propanal, CH₃CH₂CHO): Saturated aldehyde; no cis-trans isomerism.

Final answer: D