Litmus, the pH indicator, is obtained from which of the following sources?

Correct answer: (a) A lichen

Explanation

Litmus is a natural dye extracted from lichens, mainly *Roccella* species. A lichen is a partnership of a fungus and an alga (or a cyanobacterium) living together.

Litmus is purple in a neutral solution. It turns red in an acid and blue in a base, which is why blue litmus paper tests for acids and red litmus paper for bases. Red algae give agar and carrageenan, and brown algae give alginates.