Cis-trans isomerism requires different substituents on each double bond carbon

Steps:

• Geometric isomerism occurs only if each C=C carbon has two different groups attached.
• Analyze structures: propene derivatives have C=C between carbons 1 and 2, with CH₃ on carbon 3.
• For option A (1,2-trichloropropene): Cl₂C=CH-CH₃; carbon 1 has two Cl (identical).
• Confirm other options have different groups on both carbons, allowing isomerism.

Why A is correct:

• The carbon with two chlorine atoms (geminal dichloride) lacks distinct substituents, so no cis-trans forms possible.

Why the others are wrong:

• B: ClHC=CCl-CH₃; carbon 1 (H, Cl), carbon 2 (Cl, CH₃)—both different, shows isomerism.
• C: Same name as A but interpreted as ClHC=CCl-CH₂Cl variant; substituents differ, shows isomerism.
• D: ClHC=CH-CH₃; carbon 1 (H, Cl), carbon 2 (H, CH₃)—both different, shows isomerism.

Final answer: A