Which among these is not a structural isomer of the compound C4H8?

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Structural isomers share the same molecular formula but differ in how the atoms are connected. For C4H8, there are several distinct connectivities: a straight-chain alkene with a terminal double bond (but-1-ene), a branched alkene (2-methylprop-1-ene), and a cycloalkane (cyclobutane). The designation but-3-ene refers to a double bond between the third and fourth carbons in a four-carbon chain, but numbering from the other end gives but-1-ene. So but-3-ene is not a distinct structural isomer—it’s the same molecule as but-1-ene. Cyclobutane, on the other hand, has a ring structure and connectivity different from the open-chain alkenes, making it a legit structural isomer of C4H8. The key idea is that some names describe the same structure when renumbered, while others describe genuinely different connectivities.