The student tested a liquid with a red litmus paper, and the paper remained red with no change. From this observation, we can conclude that the liquid is not a base. To understand why this is the correct answer, let`s consider the properties of acids and bases.
Acids are substances that release hydrogen ions (H+) when dissolved in water. When an acid comes in contact with a red litmus paper, which is sensitive to acids, it will turn the paper blue. So, if the liquid were an acid, the red litmus paper would have turned blue.
On the other hand, bases are substances that release hydroxide ions (OH-) when dissolved in water. When a base comes in contact with a red litmus paper, it will turn the paper blue. Therefore, if the liquid were a base, the red litmus paper would have turned blue as well.
However, since the red litmus paper remained red with no change, we can conclude that the liquid is not a base. We cannot determine if it is an acid or neither an acid nor a base based on this observation alone. The correct answer is option 1: the liquid is not a base.