Answer: B. Divide each molecular formula by the highest common factor of its subscripts to get the empirical formula, then check against \(\text{C}_2\text{H}_4\text{O}\).
(1) Butanoic acid: \(\text{C}_4\text{H}_8\text{O}_2\). Divide by 2 → \(\text{C}_2\text{H}_4\text{O}\). ✓
(2) Ethanoic acid: \(\text{C}_2\text{H}_4\text{O}_2\). Divide by 2 → \(\text{CH}_2\text{O}\). ✗
(3) Ethanol: \(\text{C}_2\text{H}_6\text{O}\) — already in simplest form. Not \(\text{C}_2\text{H}_4\text{O}\) (H count is 6, not 4). ✗
(4) Ethyl ethanoate: \(\text{C}_4\text{H}_8\text{O}_2\). Divide by 2 → \(\text{C}_2\text{H}_4\text{O}\). ✓
So R could be butanoic acid (1) or ethyl ethanoate (4) — option B.
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