Among the given compounds, the most susceptible to nucleophilic attack at the carbonyl group is

Key Idea Lesser the electron density of acyl carbon atom, more will be the susceptibility of nucleophile to attack it. The $\mathrm{Cl}$ atom has strong $-I$ effect and weakest $+\mathrm{R}$ effect because of the weak $\pi$-bond between the small sized $\mathrm{C}$-atom and large sized $\mathrm{Cl}$ atom. Thus, in $\mathrm{CH}_3 \mathrm{COCl}$, acyl carbon has least electron density and hence, more susceptible for nucleophilic attack.