The correct option is Lichen.
Explanation
Indicators are substances that exhibit a change in color when introduced to acidic or basic environments. Litmus is the most widely used natural indicator for determining the acidic or basic nature of a solution.
Detailed Analysis
Litmus is a water-soluble mixture of different dyes extracted from lichens. Lichens are composite organisms arising from a symbiotic relationship between fungi and algae (or cyanobacteria) and belong to the division Thallophyta.
- Neutral State: The natural color of litmus solution is purple.
- Acidic Medium: It turns red.
- Basic Medium: It turns blue.
Mosses (Bryophyta), Ferns (Pteridophyta), and Cacti are distinct plant groups and are not sources of litmus dye.
Key Takeaway
Litmus is a natural acid-base indicator extracted from Lichens. It turns red in acids and blue in bases.