Y is C₅H₁₀, an acyclic alkene with 6 isomers Steps:

• From 10 g occupying 3.4 dm³ at room conditions (molar volume ≈24 dm³/mol), moles = 10/M, volume gives M ≈70 g/mol, so formula C₅H₁₀.
• Decolours Br₂ (unsaturation), gaseous hydrocarbon, no rings → acyclic monoalkene C₅H₁₀ (general formula CₙH₂ₙ).
• Structural isomers: pent-1-ene; cis-/trans-pent-2-ene; 2-methylbut-1-ene; 3-methylbut-1-ene; 2-methylbut-2-ene.
• Total isomers: 1 + 2 (stereoisomers) + 1 + 1 + 1 = 6, accounting for structural and stereoisomerism. Why C is correct:
• Question specifies structural and stereoisomerism; C₅H₁₀ acyclic alkenes have exactly 6 isomers per standard organic nomenclature. Why the others are wrong:
• A: Ignores cis-trans in pent-2-ene (only 4 total).
• B: Counts 5 structural isomers but omits stereoisomers.
• D: Overcounts by including rings (e.g., cyclopentane) or other unsaturations.

Final answer: C