What is the effect of detritus rich in lignin and chitin on the rate…

The correct answer is Option (2) → It slows down the rate of decomposition

Decomposition is the breakdown of dead organic matter (detritus) into simple inorganic substances. Its rate depends on the chemical make-up of the detritus and on the climate. Lignin (in wood) and chitin (in the bodies of insects and the walls of fungi) are tough substances that microbes break down only with difficulty.

Option (1) It speeds up the rate of decomposition — Incorrect. Decomposition is faster when detritus is rich in nitrogen and water-soluble substances like sugars. Lignin and chitin have the opposite effect.

Option (2) It slows down the rate of decomposition — Correct. Lignin and chitin are hard to break down, so detritus rich in them decomposes slowly.

Option (3) It has no effect on the rate of decomposition — Incorrect. The chemical make-up of detritus is one of the two main factors that control the rate of decomposition, so it does have an effect.

Option (4) It prevents decomposition entirely — Incorrect. Lignin and chitin slow decomposition down but do not stop it. Even such detritus is broken down in time by soil microbes.

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.”