A hydrocarbon of formula $\mathrm{C}{}_4\mathrm{H}{}_6$ gives a precipitate with ammoniacal silver nitrate, and on Hg(II)-catalysed hydration gives butan-2-one. The hydrocarbon is

The precipitate shows a terminal alkyne, so but-2-yne is out (although its hydration also gives butan-2-one). Hydration of $\mathrm{HC}\equiv \mathrm{C}-\mathrm{CH}{}_2\mathrm{CH}{}_3$ is Markovnikov and gives $\mathrm{CH}{}_3-\mathrm{CO}-\mathrm{CH}{}_2\mathrm{CH}{}_3$, butan-2-one. Buta-1,3-diene and cyclobutene give neither result.