Solution

Carboxylic acid derivatives tend to react via nucleophilic acyl substitution where the group on the acyl unit, $$R-C=O$$ undergoes substitution. The reactivity depends on the the ability of the substituent to function as a leaving group. $$Cl^-$$ as the electron withdrawing group creates more positive charge on $$CO$$ and is the best leaving group therefore $$RCOCl$$ is the most reactive acid derivative.