An organic compound X, $\mathrm{C}{}_4\mathrm{H}{}_8\mathrm{O}$, gives a yellow precipitate with $\mathrm{I}{}_2$/$\mathrm{NaOH}$ and the sodium salt obtained in solution contains three carbon atoms. X is

A positive iodoform test indicates $\mathrm{CH}{}_3\mathrm{CO}{}^{-}$ or $\mathrm{CH}{}_3\mathrm{CH}(\mathrm{OH}){}^{-}$. Butan-2-one, $\mathrm{CH}{}_3\mathrm{COCH}{}_2\mathrm{CH}{}_3$, loses the $\mathrm{CH}{}_3$ as $\mathrm{CHI}{}_3$ and the remaining $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{COO}{}^{-}\mathrm{Na}{}^{+}$ (sodium propanoate) has 3 carbons. Butanal, 2-methylpropanal and cyclobutanol do not have the required group.