Final answer:
The weak acid among the options provided is d. H₂SO₃ (sulfurous acid), as it does not ionize completely in water, unlike the other strong acids listed.
Step-by-step explanation:
Among the options provided, d. H₂SO₃ (sulfurous acid) is classified as a weak acid. In contrast, the other acids listed (HNO₃, HI, HBr, HClO₄) are known as strong acids, which dissociate completely in water. Sulfurous acid does not ionize completely and therefore is a weak acid. Examples of other weak acids include H₂PO (phosphoric acid), CH₃COOH (acetic acid), and H₂CO₃ (carbonic acid). In contrast, strong acids such as H₂SO₄ (sulfuric acid) dissociate fully to release more hydrogen ions in solution.