Answer: B. Isomers share the same molecular formula but have different structures. Compute the molecular formula of each compound:
(1) Butanol = \(\text{C}_4\text{H}_9\text{OH} = \text{C}_4\text{H}_{10}\text{O}\)
(2) Butanoic acid = \(\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH} = \text{C}_4\text{H}_8\text{O}_2\)
(3) Propanoic acid = \(\text{CH}_3\text{CH}_2\text{COOH} = \text{C}_3\text{H}_6\text{O}_2\)
(4) Ethyl ethanoate = \(\text{CH}_3\text{COOCH}_2\text{CH}_3 = \text{C}_4\text{H}_8\text{O}_2\)
Compounds 2 and 4 share \(\text{C}_4\text{H}_8\text{O}_2\) — they are isomers (a carboxylic acid and an ester respectively, with the same atoms arranged differently).
Compounds 1 and 3 each have unique formulas. Option B (2 and 4).
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