An optically active alkyl bromide \( \mathrm{C_{4}H_{9}Br} \), reacts with ethanolic \( \mathrm{KOH} \) to form major compound [A] which reacts with bromine to give compound [B]. Compound [B] reacts with ethanolic \( \mathrm{KOH} \) and sodamide to give compound [C]. One molecule of water adds to compound [C] on warming with mercuric sulphate and dilute sulphuric acid at \( 333 \, \text{K} \) to form compound [D]. The functional group in compound D will be confirmed by: