Concept:Structural isomers have the same molecular formula but different arrangement of atoms.Explanation:2-methylprop-1-ene has the molecular formula C4H8.It is an alkene with the same formula as butene isomers.The straight-chain alkenes with formula C4H8 are but-1-ene and but-2-ene.But-2-ene, CH3CH=CHCH3, is a structural isomer of 2-methylprop-1-ene.Options A and C are alkynes with different formulas, and Option B is the same compound misnamed.Answer:D. But-2-ene