Which of the following animals is called farmer's friend?

In agriculture and soil-science teaching, the epithet "farmer's friend" is conventionally given to the organism whose everyday, direct activity most visibly improves soil fertility, aeration, and drainage for crop growth, as distinct from creatures that help crops only indirectly, such as through pollination.

Earthworms burrow continuously through the soil, swallowing soil particles and decaying organic matter as they move, and excreting this material as nutrient-rich castings. Their tunnels loosen compacted soil, improve aeration, and let water and plant roots penetrate more easily; at a larger scale, this same earthworm-driven breakdown of organic waste into castings is deliberately harnessed in vermicomposting units to produce a nutrient-rich natural fertilizer. This direct, continuous soil enrichment is why earthworms are conventionally given the title of the farmer's friend in agriculture and school curricula.

• Ants are soil-dwelling insects whose day-to-day activity centers on foraging for food and building colonies rather than on enriching soil; some ant species even protect aphid colonies that damage crops.

• Bees help agriculture mainly by pollinating flowers, which raises crop yield, a pollination role rather than the direct soil-enrichment role this conventional epithet describes.

• Butterflies, like bees, act mainly as pollinators as adults, while their caterpillar stage commonly feeds on host-plant leaves, again a role distinct from direct soil enrichment.

So, of the animals listed, the earthworm's continuous burrowing-and-casting activity is the standard basis for calling it the farmer's friend in agriculture and school-level teaching.