Isomers are compounds that have the same molecular formula but different structural formulas. Among the given options, the pair of organic compounds that are isomers are But-1-ene and But-2-ene because they have the same molecular formula (C4H8) but different arrangements of atoms in their structure. In But-1-ene, the double bond is between the first and second carbon atoms while in But-2-ene, the double bond is between the second and third carbon atoms. Thus, the correct answer is But-1-ene and But-2-ene.