Sodium hydroxide (NaOH) is a strong base. Bases turn red litmus paper blue.

Here's why:

* Litmus paper is a pH indicator, meaning it changes color depending on the acidity or basicity of a solution.

* Red litmus paper is used to test for bases.

* Sodium hydroxide is a base.

Therefore, when sodium hydroxide is added to red litmus paper, the paper will turn blue, indicating the presence of a base.