A solution turns red litmus blue, which of the following is most likely to be the pH value of the solution?

pH = 10

The correct answer is option 4. pH = 10.

Litmus paper is a simple indicator commonly used to test the acidity or alkalinity of a solution. Litmus paper changes color based on the pH of the solution:

Red litmus paper turns blue in a basic (alkaline) solution.

Blue litmus paper turns red in an acidic solution.

Given that the solution turns red litmus paper blue, it indicates that the solution is basic or alkaline.

The pH scale measures the acidity or alkalinity of a solution, ranging from 0 to 14. A pH value below 7 indicates acidity, 7 is neutral, and values above 7 indicate alkalinity.

In the options provided:

pH = 1: Highly acidic.

pH = 4: Moderately acidic.

pH = 5: Slightly acidic.

pH = 10: Moderately alkaline.

Since the solution turns red litmus paper blue, it is most likely to have a pH in the alkaline range. Therefore, pH = 10 is the most appropriate choice among the options.