What is an example of mutualism?

The correct answer is Option (1) → Lichens consisting of fungi and algae

In mutualism, both interacting species benefit. A lichen is a close partnership between a fungus and an alga or cyanobacterium in which each partner gains. The other three pairs are the book’s examples of commensalism, where one partner gains and the other gets no apparent benefit.

Option (1) Lichens consisting of fungi and algae — Correct. Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. Both partners benefit.

Option (2) Cattle egret foraging near grazing cattle — Incorrect. The egret eats the insects that the moving cattle flush out of the grass, but the cattle gain nothing. This is commensalism.

Option (3) Barnacles growing on the back of a whale — Incorrect. The barnacles get a place to live, while the whale derives no apparent benefit. This is commensalism.

Option (4) Orchid growing as an epiphyte on a mango branch — Incorrect. The orchid gets support on the mango branch, while the mango tree derives no apparent benefit. This is commensalism.

NCERT Extract:
Population Interactions – “… (iv) Commensalism: This is the interaction in which one species benefits and the other is neither harmed nor benefited. An orchid growing as an epiphyte on a mango branch, and barnacles growing on the back of a whale benefit while neither the mango tree nor the whale derives any apparent benefit. The cattle egret and grazing cattle in close association, a sight you are most likely to catch if you live in farmed rural areas, is a classic example of commensalism.”
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.”