Q.The phenomenon by which a bird such as the koel lays its eggs in the nest of another species (such as the crow), which then rears the koel's young, is called:
(A) Commensalism
(B) Brood parasitism
(C) Predation
(D) Mutualism

This is a specialised form of parasitism, not predation or mutualism.

The scenario described — one bird species (the koel) laying its eggs in the nest of another species (the crow), which then rears the koel's chick — is the textbook definition of brood parasitism, a specialised form of parasitism in which the host's own parental investment is exploited to raise the parasite's offspring. Commensalism (option A) would require the crow to be unaffected, but the crow's own reproductive success is actually reduced by having to raise the koel chick, so this is not commensalism. Predation (option C) would require the koel to kill and eat the crow or its eggs, which is not what happens. Mutualism (option D) would require both species to benefit, but only the koel benefits here.

The key phrase to watch for is "lays its eggs in the nest of another species, which then rears the young" — this exact scenario always identifies brood parasitism, regardless of which two bird species are named.

Do not select commensalism just because the crow is not physically killed or eaten — the crow's OWN reproductive success genuinely suffers (its own eggs/chicks are often neglected or outcompeted), so the crow is harmed, ruling out commensalism.