The correct answer is: a) H2SO3.
Explanation: A diprotic acid is one that can donate two protons (H+) per molecule in a reaction. Among the options given, H2SO3 (sulfurous acid) is a weak diprotic acid, meaning it does not completely dissociate in solution. In contrast, the other acids listed—HClO4 (perchloric acid), HNO3 (nitric acid), H3PO4 (phosphoric acid), and H2SO4 (sulfuric acid)—are either strong acids or stronger diprotic acids. Strong acids fully dissociate in aqueous solutions, while weak acids only partially dissociate, which is characteristic of H2SO3.