In the collapse of the tetrahedral intermediate formed from $\mathrm{CH}{}_3-\mathrm{CO}-\mathrm{X}$ and a nucleophile, which group X is lost most easily?

The better leaving group is the weaker base, i.e. the conjugate base of the stronger acid. HCl (pKa about $-7$) is far stronger than $\mathrm{CH}{}_3\mathrm{COOH}$ (4.76), $\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{OH}$ (about 16) and $\mathrm{NH}{}_3$ (about 38), so $\mathrm{Cl}{}^{-}$ leaves most easily.