उत्तर : 4. "लाइकेन से"
व्याख्या :

Litmus is obtained from lichens, which are composite organisms formed from a symbiotic relationship between a fungus and an alga or cyanobacterium. Litmus is a water-soluble mixture of different dyes extracted from lichens, primarily Roccella tinctoria and Roccella fuciformis. It is commonly used as a pH indicator in laboratories to test the acidity or alkalinity of solutions.