Assertion (A): Earthworms are useful to man. Reason (R): Earthworms help in Nitrogen Fixation in plants.

Answer: (a) Both A and R are true, and R is the correct explanation of A

Earthworms make soil fertile; their gut and casts support nitrogen-fixing bacteria, raising the nitrogen available to plants — this is the link UPSC accepts.

The idea

An earthworm eats its way through the soil. It swallows soil and dead leaves, digests what it can and passes out the rest as fine pellets called casts, which are richer in plant nutrients than the soil around them. Its burrows let air and water in.

Swallows soil and leaf litter → Grinds and digests it → Passes out nutrient-rich casts → Burrows loosen the soil → Soil microbes, including nitrogen-fixers, thrive

Why the others are wrong

(b) Treats nitrogen as unrelated to their usefulness — but soil fertility is their main use.

(c) Earthworms don’t fix nitrogen themselves, which makes R sound false. But they HELP fixation by feeding nitrogen-fixing bacteria.

(d) Earthworms are clearly useful.

The trap

“Help in nitrogen fixation” is not the same as “fix nitrogen”. Read the verb.

Also know

This is why the earthworm is called the farmer’s friend. Making compost with earthworms is vermicomposting. Charles Darwin spent years studying how worms build soil.