The correct answer is option 1, red litmus.

Red litmus paper is used to test for acids. It turns blue in the presence of an alkaline solution (a basic solution) and remains red in the presence of an acidic solution.

In this question, the test tubes contain water (H2O) and aqueous NaOH. NaOH is sodium hydroxide, which is a strong base and classified as an alkaline solution. When sodium hydroxide is dissolved in water, it forms hydroxide ions (OH-) which contribute to the alkalinity of the solution.

Therefore, when red litmus paper is dipped into the test tube, it will turn blue, indicating that it is an alkaline solution (a base).

Option 2 (blue litmus) would not be effective in differentiating between the two solutions because blue litmus paper turns red in the presence of an acid, but it stays blue in alkaline solutions.

Options 3 (Na2CO3, sodium carbonate) and 4 (HCl, hydrochloric acid) are not relevant to differentiating between water and aqueous NaOH, as they are different compounds and would not provide any information about the alkalinity of the solution in the