Which substance will cause litmus paper to turn red?
a. a?

Acids will turn litmus paper to turn red.

Explanation:

Litmus paper is an indicator which is used to detect if the solution is acidic or basic.

There are 2 types of litmus paper: Blue litmus paper and Red litmus paper.

Blue litmus paper has no effect if the solution is basic but will turn Red if the solution is acidic.

Red litmus paper has no effect if the solution is acidic but will turn blue if the solution is basic.

Hence, only acidic solutions will cause litmus paper to turn red.