C4H8 alkenes with one double bond

Steps:

• Hydrogen mass percent 14.3% gives empirical formula CH2; combined with Br2 addition data, molecular formula is C4H8 (molar mass 56 g/mol from 0.28 g hydrocarbon : 0.79 g Br2 ≈ 1:1 mole ratio for one double bond).
• C4H8 indicates one degree of unsaturation, consistent with alkenes adding one Br2 molecule.
• Structural isomers: 1-butene, 2-butene, 2-methylpropene (cycloalkanes excluded as they do not undergo addition).
• Stereoisomers: 2-butene has cis and trans forms; others have none, totaling four isomers.

Why C is correct:

• Four C4H8 alkenes (1-butene, cis-2-butene, trans-2-butene, 2-methylpropene) each add one Br2, matching the reaction stoichiometry per alkene addition rule.

Why the others are wrong:

• A: Only one would ignore structural and stereoisomers.
• B: Misses one structural or stereo variant.
• D: Includes non-adding cycloalkanes like cyclobutane, violating addition reaction.

Final answer: C