During decomposition, which step leads to the accumulation of a dark-…

The correct answer is Option (4) → Humification

Decomposition breaks down dead plant and animal remains (detritus) into carbon dioxide, water and nutrients. It has five steps: fragmentation, leaching, catabolism, humification and mineralisation. Each step does a different job, and only one of them builds up humus.

Option (1) Fragmentation — Incorrect. In fragmentation, detritivores such as the earthworm break detritus into smaller particles. No humus is formed in this step.

Option (2) Leaching — Incorrect. In leaching, water-soluble inorganic nutrients go down into the soil and get precipitated as unavailable salts.

Option (3) Catabolism — Incorrect. In catabolism, bacterial and fungal enzymes degrade detritus into simpler inorganic substances.

Option (4) Humification — Correct. Humification leads to the build-up of humus, a dark coloured amorphous substance. Humus resists microbial action, decomposes very slowly and, being colloidal, serves as a reservoir of nutrients.

NCERT Extract:
Decomposition – “… The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralisation.”
Decomposition – “… Humification and mineralisation occur during decomposition in the soil. Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients.”