If a solution has pH 10, what effect will it have on red litmus paper?

Correct answer: It will turn blue

A pH greater than 7 indicates a basic or alkaline solution. Because the given pH is 10, the solution is basic. A base changes red litmus paper to blue, so option A is correct. Red litmus remaining red would suggest no basic change, while yellow or colourless are not the characteristic observations for this indicator test. The conclusion follows directly from the pH scale and litmus behaviour.