The correct answer is C) concentrated nitric acid. Concentrated nitric acid reacts dangerously with vinyl chloride, as it can cause a violent reaction and release toxic fumes. This is an important safety consideration when handling or storing vinyl chloride. The other options are incorrect because: A) saltwater does not react dangerously with vinyl chloride, B) alkalies (basic substances) are generally not incompatible with vinyl chloride, and D) organic acids, while they may react with vinyl chloride, do not have the same level of hazard as concentrated nitric acid.