Final answer:
Trans-but-2-ene is the most stable isomer among the given options due to lower steric hindrance between the methyl groups positioned on opposite sides of the double bond.
Step-by-step explanation:
The question of which isomer is most stable among But-1-ene, cis-but-2-ene, and trans-but-2-ene concerns the concept of geometric isomerism in alkenes. In this type of isomerism, the isomers have the same molecular formula but differ in the spatial arrangement of the atoms around the double bond. Trans-but-2-ene is generally more stable than cis-but-2-ene due to lower steric hindrance between the two methyl groups, which are positioned on opposite sides of the double bond in the trans-isomer. But-1-ene does not exhibit cis-trans isomerism since the double bond is at the end of the molecule. Therefore, the most stable isomer among the options provided is trans-but-2-ene.