Which of the following is a source for extracting litmus?

lichens 

The correct answer is option 3. lichens.

Litmus is a natural dye derived from lichens, specifically certain species of lichens. Lichens are symbiotic associations between fungi and algae or cyanobacteria. Litmus is obtained from lichens belonging to the genus Roccella, particularly Roccella tinctoria and related species.

Here is how litmus is extracted from lichens:

1. Harvesting Lichens: The lichens are harvested, usually from rocks or other substrates where they grow naturally. Roccella tinctoria, in particular, has been historically used for litmus production.

2. Processing:  The harvested lichens are processed to extract the litmus dye. This involves several steps, including fermentation and extraction processes to release and isolate the color-producing compounds.

3. Drying:  After extraction, the litmus solution is often dried to obtain the final litmus product. The dried litmus can take the form of paper strips or powder.

Litmus is known for its ability to change color in response to changes in acidity or alkalinity (pH). It turns red under acidic conditions and blue under alkaline conditions. Litmus is widely used as a pH indicator in laboratories, educational settings, and various industries to test the acidity or alkalinity of solutions.

While litmus can be derived from lichens, it's worth noting that synthetic litmus paper is also available, providing a convenient and standardized pH indicator for various applications.