Lichens are an example of which type of population interaction?

The correct answer is Option (2) → Mutualism

A lichen is made of two partners, a fungus and an alga (or a cyanobacterium), living in close association. Both partners gain, so a lichen is an example of mutualism.

Option (1) Parasitism — Incorrect. In parasitism one species lives at the expense of the other and harms it. In a lichen neither partner is harmed.

Option (2) Mutualism — Correct. Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. The interaction confers benefits on both partners.

Option (3) Commensalism — Incorrect. In commensalism only one species benefits and the other is neither harmed nor benefited. In a lichen both partners benefit.

Option (4) Amensalism — Incorrect. In amensalism one species is harmed and the other is unaffected. No partner in a lichen is harmed.

NCERT Extract:
Population Interactions – “… (v) Mutualism: This interaction confers benefits on both the interacting species. Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. Similarly, the mycorrhizae are associations between fungi and the roots of higher plants.”