Geometric isomerism can exist in

Correct answer: hex-3-ene.

Explanation: Geometric (cis/trans or E/Z) isomerism in alkenes requires a C=C double bond where each double-bonded carbon bears two different substituents (restricted rotation + different groups).

• hex-3-ene — The C3=C4 carbons each have H and an ethyl group (different on each carbon) ⇒ cis/trans possible.
• hexane — Saturated (no C=C) ⇒ no geometric isomerism.
• prop-1-ene — One C of the double bond is CH₂ (two identical H) ⇒ no geometric isomerism.
• 3-methylbut-1-ene — Terminal alkene (CH₂=), first C has two H ⇒ no geometric isomerism.