In some shallow South American lakes, visiting flamingoes and residen…

The correct answer is Option (4) → Totally unrelated species could also compete for the same resource.

Competition is often thought to happen only between closely related species. The book says this is not entirely true. Flamingoes are birds and fishes are fishes, two groups that are not at all closely related, yet both feed on the zooplankton of the same shallow lakes. The example shows that totally unrelated species can also compete for the same resource.

Option (1) Two closely related species competing for the same resources cannot co-exist indefinitely. — Incorrect. This is Gause’s principle about closely related species. The flamingoes and fishes are unrelated, and the example says nothing about one of them being eliminated.

Option (2) Species facing competition co-exist by partitioning the resource between them. — Incorrect. Resource partitioning lets competitors avoid competition, for example by feeding at different times. The example says the two groups do compete for their common food.

Option (3) A restricted species expands its range when the competing species is removed. — Incorrect. This describes competitive release, which is seen when a superior competitor is experimentally removed. Nothing is removed in the flamingo example.

Option (4) Totally unrelated species could also compete for the same resource. — Correct. Birds and fishes are far apart in relationship, but they share one food, zooplankton, and so they compete for it.

NCERT Extract:
Population Interactions – “… It is generally believed that competition occurs when closely related species compete for the same resources that are limiting, but this is not entirely true. Firstly, totally unrelated species could also compete for the same resource. For instance, in some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton in the lake.”