The substance that turns red in the presence of acid and blue in the presence of alkali is litmus paper.

Here's why:

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

* Acidic solutions have a pH lower than 7, and litmus paper turns red in their presence.

* Basic (alkaline) solutions have a pH higher than 7, and litmus paper turns blue in their presence.

This color change makes litmus paper a simple and useful tool for quickly determining whether a solution is acidic or basic.