Which of the following is NOT correctly matched?

1. A

   Acid present in - Acetic acid vinegar

   Vinegar contains acetic acid. The pair is correctly matched. It is not the unmatched row. Everyday sourness of vinegar is acetic, not the error asked.

2. B

   Compound present - Calcium phosphate in bones

   Bones contain calcium phosphate (hydroxyapatite) as the main mineral. The pair is correctly matched. This is not the wrong row.

3. C

   Souring of milk - Nitric acid

   (c) Official key: Souring of milk is due to lactic acid produced by lactose fermentation, not nitric acid. Nitric acid is a mineral acid unrelated to milk spoilage. This is the pair that is not correctly matched.

4. D

   Acid present in - Hydrochloric acid gastric juice

   Gastric juice contains hydrochloric acid. The pair is correctly matched. HCl in the stomach is standard physiology, not the error.

Summary. Official key is (c). Acetic acid in vinegar, calcium phosphate in bones, and HCl in gastric juice are correctly paired. Milk sours by lactic acid, not nitric acid, so that row is unmatched. The keyed incorrect match is souring of milk — nitric acid.