Why are earthworms considered farmer's friends?

Due to their role in fragmentation of detritus and loosening of soil.

The correct answer is (a) they help in the fragmentation of detritus and loosening of soil.

Earthworms are referred to as farmer's friends because they provide several benefits to the soil and agriculture. One of their key roles is in the fragmentation of detritus, which involves breaking down organic matter into smaller particles. This process aids in the decomposition of organic material and helps in nutrient recycling.

Additionally, earthworms contribute to the loosening of the soil through their burrowing activities. As they tunnel through the soil, they create channels and passages, which improve soil aeration and drainage. The loosening of the soil by earthworms promotes root growth, nutrient absorption, and water infiltration.

Options (b) and (c), which state that earthworms help in immobilization of nutrients inside the soil and reduce the rate of decomposition, are incorrect. Earthworms do not immobilize nutrients but rather enhance nutrient availability by breaking down organic matter and facilitating nutrient release. They also accelerate the decomposition process rather than reducing it.