The given question is based on the concept of litmus paper and its reaction with acids and bases. Litmus paper is a commonly used indicator to determine whether a substance is an acid or a base.
When a substance is acidic, it turns the blue litmus paper red. On the other hand, when a substance is basic, it turns the red litmus paper blue.
In this scenario, the student tested a liquid with a red litmus paper and observed no change in its color, which means that the red litmus paper remained red.
Based on this observation, we can conclude that the liquid is not a base. This eliminates option 1, which suggests that the liquid must be pure water, as pure water is neutral and does not affect the color of the litmus paper.
However, we cannot assume that the liquid is an acid, as there was no change in the color of the red litmus paper. Therefore, option 2, suggesting that the liquid must be an acid, is incorrect.
Option 4 states that the liquid is neither a base nor an acid. This option does not directly contradict the observation made by the student, and hence, it is a plausible answer.
In conclusion, the correct answer is option 3, which states that