The process of mineralisation by microorganisms helps in the release…

The correct answer is Option (3) → inorganic nutrients from humus.

Option (1) inorganic nutrients from detritus and formation of humus. — Incorrect. Humus is formed by humification, not by mineralisation. The breakdown of detritus into simpler inorganic substances by bacterial and fungal enzymes is catabolism.

Option (2) organic nutrients from humus. — Incorrect. Mineralisation releases inorganic nutrients that plants can take up, not organic ones.

Option (3) inorganic nutrients from humus. — Correct. In decomposition, humus is first formed (humification) and then degraded by some microbes. This mineralisation releases inorganic nutrients from humus back into the soil.

Option (4) organic and inorganic nutrients from detritus. — Incorrect. Detritus is broken down by fragmentation, leaching and catabolism. Mineralisation works on humus and releases only inorganic nutrients.

NCERT Extract:
Decomposition – “… Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This process is called as catabolism. It is important to note that all the above steps in decomposition operate simultaneously on the detritus (Figure 12.1). 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. The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.”