An optically active alkyl bromide $\mathrm{C}_4 \mathrm{H}_9 \mathrm{Br}$, reacts with ethanolic KOH to form major compound $[\mathrm{A}]$ which reacts with bromine to give compound $[\mathrm{B}]$. Compound $[\mathrm{B}]$ reacts with ethanolic 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 K to form compound $[\mathrm{D}]$. The functional group in compound D will be confirmed by :