Which of the following examples does not illustrate commensalism?

The correct answer is Option (4) → Brood parasitism in birds

Commensalism is an interaction in which one species benefits and the other is neither harmed nor benefited (+, 0). Parasitism is different: one species benefits and the other, the host, is harmed (+, −).

Option (1) Orchid and the tree on which it grows — An example of commensalism. The orchid grows as an epiphyte on a mango branch and gets support and light. The tree gets no benefit and suffers no harm.

Option (2) Cattle egret and grazing cattle — An example of commensalism. Cattle stir up insects as they graze, and the egrets catch them easily. The cattle neither gain nor lose.

Option (3) Sea anemone and clown fish — An example of commensalism. The clown fish lives among the stinging tentacles of the sea anemone and is protected from predators. The anemone gets no apparent benefit.

Option (4) Brood parasitism in birds — Not an example of commensalism; it is parasitism. The parasitic bird, such as the cuckoo (koel), lays its eggs in the nest of a host bird, such as the crow, and the host incubates them. The host loses its effort and resources, so it is harmed.

NCERT Extract:
Population Interactions – “… Brood parasitism in birds is a fascinating example of parasitism in which the parasitic bird lays its eggs in the nest of its host and lets the host incubate them. … (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. … Another example of commensalism is the interaction between sea anemone that has stinging tentacles and the clown fish that lives among them.”