In the grasslands, trees do not replace the grasses as a part of an ecological succession because of

Answer: (c) Water limits and fire

Too little water for tree saplings and repeated fires that kill them before they grow bark. Grasses re-sprout from underground roots; many trees can too.

The idea

Succession stops at a stable climax. In grasslands the climax is set by climate (low rain) and disturbance (fire, grazing) — a “fire climax”.

Bare land → Grasses and herbs → Shrubs → Trees (if rain allows) → In grassland: low rain + fire stop the sequence at grass

Why the others are wrong

(a) Pests attack trees everywhere, including forests. They don’t explain whole biomes staying grassland.

(b) Grasslands have full sunlight and often rich soils (the prairies and steppes are breadbaskets).

(d) (c) is the textbook reason.

The trap

“Paucity of nutrients” sounds scientific, but grassland soils (chernozem, mollisols) are among the most fertile on Earth.

Also know

Succession on bare rock, where no soil existed, is primary; lichens are the pioneers. Succession after a fire or on an abandoned field, where soil remains, is secondary and is faster. The final stable community is the climax.