Answer: C. Q produces a gas (effervescence) with aqueous sodium carbonate. This is the standard test for a carboxylic acid: 2RCOOH + Na₂CO₃ → 2RCOONa + H₂O + CO₂. The gas given off is carbon dioxide, so Q is a carboxylic acid, which must contain the –COOH group and so have at least two oxygen atoms.
Option C is correct: if Q has the empirical formula C₂H₄O, its molecular formula is a multiple of this — C₄H₈O₂, which is butanoic acid, a carboxylic acid. So Q could indeed have this empirical formula.
Option D gives the molecular formula C₂H₄O, which has only one oxygen atom — that is ethanal (an aldehyde), not an acid, so it would not react with sodium carbonate. Option A is wrong (ethanol must be oxidised, not reduced, to give an acid); option B is wrong (esters form from an alcohol and an acid, but Q is itself the acid).
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