Lichens are an example of -

The correct answer is Option (3) → Mutualism

Option (1) Commensalism — Incorrect. In commensalism one gains and the other is neither harmed nor helped (+/0), e.g. an orchid growing on a mango branch.

Option (2) Parasitism — Incorrect. In parasitism one lives on the other and harms it (+/−), e.g. the human liver fluke.

Option (3) Mutualism — Correct. In a lichen the alga or cyanobacterium makes food, and the fungus gives shelter and water. Both partners gain (+/+).

Option (4) Competition — Incorrect. In competition both species lose, because they share a limited resource (−/−).

NCERT Extract:
Population Interactions – …