Answer: D. Count structural isomers for each formula.
(1) \(\text{C}_3\text{H}_8\) (propane): only one possible structure, \(\text{CH}_3\text{CH}_2\text{CH}_3\). 1 isomer. ✗
(2) \(\text{C}_4\text{H}_8\) fits \(\text{C}_n\text{H}_{2n}\) — alkene or cycloalkane. Alkene isomers alone: but-1-ene, but-2-ene (cis and trans count as a pair) and methylpropene — at least 3 (more if cyclic structures are counted). Not exactly 2. ✗
(3) \(\text{C}_4\text{H}_{10}\) (butane): two structures, butane (\(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3\)) and 2-methylpropane (\((\text{CH}_3)_3\text{CH}\)). Exactly 2 isomers. ✓
Only 3 has exactly two isomers — option D.
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