The rate of decomposition is controlled by:

The correct answer is Option (3) → Both chemical composition of detritus and climatic factors

Option (1) Chemical composition of detritus only — Incorrect. Composition matters: detritus rich in lignin and chitin breaks down slowly, and detritus rich in nitrogen and sugars quickly. But it is not the only factor.

Option (2) Climatic factors only — Incorrect. Climate matters too: warm and moist conditions speed decomposition, while low temperature and lack of oxygen slow it. It also is not the only factor.

Option (3) Both chemical composition of detritus and climatic factors — Correct. NCERT names both factors together: the chemical composition of the detritus and the climatic factors. An option naming only one of them is incomplete.

Option (4) Atmospheric pressure — Incorrect. NCERT does not name atmospheric pressure as a factor in decomposition.

NCERT Extract:
Decomposition – “… Decomposition is largely an oxygen-requiring process. The rate of decomposition is controlled by chemical composition of detritus and climatic factors. In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker, if detritus is rich in nitrogen and water-soluble substances like sugars.”