What is the term for the process in which water-soluble inorganic nutrients move down into the soil horizon and precipitate as unavailable salts?

Leaching

The correct answer is (b) leaching.

Leaching is the process by which water-soluble inorganic nutrients, present in the detritus or organic matter, move down into the soil horizon. As water percolates through the soil, it carries these nutrients with it. In some cases, these nutrients can get precipitated as salts, becoming unavailable for uptake by plants and other organisms.

Option (a) fragmentation refers to the process of breaking down detritus into smaller particles by detritivores, such as earthworms.

Option (c) catabolism refers to the process of breaking down complex organic matter into simpler inorganic substances through the action of bacterial and fungal enzymes.

Option (d) mineralisation refers to the process of releasing inorganic nutrients from organic matter through the decomposition activities of microbes.

Therefore, the correct choice is (b) leaching, as it specifically describes the process of water-soluble inorganic nutrients moving down into the soil horizon and getting precipitated as unavailable salts.