Among the given options, the weak diprotic acid is d) H2CO3.
Explanation: A diprotic acid is an acid that can donate two protons (H+) per molecule in a solution. Acetic acid (CH3COOH) is a weak monoprotic acid, meaning it only donates one proton and does not fit the criteria. Phosphoric acid (H3PO4) and sulfuric acid (H2SO4) are both diprotic acids, but they are stronger acids in nature. H2SO4 is a strong acid that completely dissociates in solution, while H3PO4 is a weak acid but not weaker than H2CO3. On the other hand, carbonic acid (H2CO3) is indeed a weak diprotic acid, as it does not fully dissociate in water. Thus, H2CO3 is the correct answer.