Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients and the process is called decomposition. Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter, constitute detritus, which is the raw material for decomposition.

Assertion: Decomposition is the physical and chemical breakdown of complex organic matter into simple inorganic substances.

Reason:   Humification is the process of formation of humus from detritus or organic remains.

 Both Assertion (A) and Reason (R) are the true but Reason (R) is not a correct explanation of Assertion (A).

Option 2 is correct .  Both Assertion (A) and Reason (R) are the true but Reason (R) is not a correct explanation of Assertion (A).

Decomposition is a biological process that includes the physical breakdown and biochemical transformation of complex organic molecules of dead material into simpler organic and inorganic molecules. The process is a part of the nutrient cycle and is essential for recycling the finite matter that occupies physical space in the biosphere. Bodies of living organisms begin to decompose shortly after death.  Decomposition of organic matter is largely a biological process that occurs naturally. Its speed is determined by three major factors:

soil organisms, the physical environment and the quality of the organic matter.

 Successive decomposition of dead material and modified organic matter results in the formation of a more complex organic matter called humus. This process is called humification. Humus affects soil properties. As it slowly decomposes, it colours the soil darker; increases soil aggregation and aggregate stability; increases the CEC (the ability to attract and retain nutrients); and contributes N, P and other nutrients.

Humus denominates the fraction of soil organic matter that is amorphous and without the "cellular cake" structure characteristic of plants, micro-organisms or animals. Humus significantly affects the bulk density of soil and contributes to its retention of moisture and nutrients.