A compound $\mathrm{C}{}_4\mathrm{H}{}_8\mathrm{O}$ gives an orange precipitate with 2,4-DNP, no silver mirror with Tollens reagent, and a yellow precipitate with $\mathrm{NaOH}/\mathrm{I}{}_2$. It is

2,4-DNP shows a carbonyl group, negative Tollens shows a ketone, and the iodoform test shows $\mathrm{CH}{}_3\mathrm{CO}{}^{-}$. Among the isomers only butan-2-one, $\mathrm{CH}{}_3\mathrm{COCH}{}_2\mathrm{CH}{}_3$, fits. Butanal and 2-methylpropanal are aldehydes, and but-3-en-1-ol has no carbonyl.