Explanation:

The pH scale measures the acidity or basicity of a solution. The lower the pH value, the more acidic the solution. The pH of pure water is 7, which is neutral. Concentrated sugar solution is also neutral, with a pH of around 7. Sodium hydroxide is a strong base, with a pH of around 14. Gastric fluid is the acidic fluid produced in the stomach to help digest food. It has a pH of around 1.5, which is the lowest of the given options.