Answer: C (C₃H₆). Work backwards from Z. Z is propanoic acid, CH₃CH₂COOH, which has three carbon atoms. Carboxylic acids are made by oxidising primary alcohols, so Y must be propan-1-ol, CH₃CH₂CH₂OH (also three carbons).
Alcohols are made by adding steam (hydration) across the C=C of an alkene, so X must be a three-carbon alkene: propene, CH₂=CHCH₃, with the formula C₃H₆.
C₂H₄ (ethene) would give ethanol and only two carbons; C₃H₈ is propane, an alkane that does not add steam; and C₂H₆ is ethane. The carbon count must stay at three throughout, so X is propene, C₃H₆.
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