The colour of bromine water disappearing when added to a carbon compound is a test for the presence of a:

1. Unsaturated compounds undergo an addition reaction with bromine at room temperature. 2. As bromine adds across the bond, its colour disappears. 3. This instant colour change detects a multiple (double or triple) bond. 4. Rule out B: a single bond is saturated and does not decolourise bromine. 5. Rule out A: a carboxylic acid group is detected by its acidity, not by bromine. 6. Rule out D: a hydroxyl group reacts with sodium, not by decolourising bromine. _Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 9 "Carbon Compounds", p.134_