Which of the following statements about predators is incorrect?

The correct answer is Option (3) → For plants, herbivores are the parasites, and plants have evolved a variety of morphological and chemical defences against them.

The question asks for the incorrect statement about predators. Predators pass the energy fixed by plants up the food chain, and they also keep prey numbers in check. In the book's wide sense, an animal eating a plant is also a predator.

Option (1) Predators keep prey populations under control. — A correct statement. Predators remove prey and so regulate prey numbers.

Option (2) But for predators, prey species could achieve very high population densities and cause ecosystem instability. — A correct statement. Without predators, prey numbers would rise very high and upset the ecosystem.

Option (3) For plants, herbivores are the parasites, and plants have evolved a variety of morphological and chemical defences against them. — The incorrect statement. For plants, herbivores are the predators, not parasites. The second half is true: plants have thorns and chemicals against herbivores.

Option (4) Some exotic species introduced into a geographical area become invasive and spread fast because the invaded land does not have their natural predators. — A correct statement. The prickly pear cactus spread over Australia until a cactus-feeding moth, its natural predator, was brought in.

NCERT Extract:
Population Interactions – “… They keep prey populations under control. But for predators, prey species could achieve very high population densities and cause ecosystem instability. When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast because the invaded land does not have its natural predators.”
Population Interactions – “… For plants, herbivores are the predators. Nearly 25 per cent of all insects are known to be phytophagous (feeding on plant sap and other parts of plants). … Plants therefore have evolved an astonishing variety of morphological and chemical defences against herbivores.”