Some species of plants are insectivorous. Why?

Answer: (b) They are adapted to grow in nitrogen deficient soils and thus depend on insects for sufficient nitrogenous nutrition.

They grow in nitrogen-poor soils (bogs, wet rocks), so they digest insects for nitrogen.

The idea

An insect-eating plant is green and makes its own food like any other plant. What it lacks is nitrogen, because it grows in bogs, wet rocks and poor sandy soil where nitrogen is scarce. An insect’s body is rich in protein, which means nitrogen. So the plant traps insects as a fertiliser, not as a meal.

Soil is poor in nitrogen → Plant traps an insect → Digests it with enzymes → Absorbs the nitrogen → Still makes its food by photosynthesis

Why the others are wrong

(a) They have green leaves and photosynthesise normally — they often grow in open sunny bogs.

(c) They don’t depend on insects for vitamins.

(d) They are not living fossils; carnivory evolved many times.

The trap

(a) assumes they eat insects for food energy. They make their own food; insects add nitrogen.

Also know

Different plants use different traps: Drosera has sticky leaves, the Venus flytrap has snap traps, and Utricularia uses suction bladders. Nepenthes khasiana is a pitcher plant native to Meghalaya. These traps are modified leaves. Bladderworts can catch tiny aquatic animals, so the wider term carnivorous plant is more accurate than insect-eating plant.