The organism is found in souls rich in a,Humus. b,Clay. c,sand d,mud

Organisms, especially decomposers like bacteria, fungi, and various invertebrates (e.g., earthworms, insects), thrive in soils that are rich in organic matter. This organic matter, when decomposed, forms humus. Humus provides essential nutrients, improves soil structure, and helps retain water and air, creating an ideal environment for a wide range of soil organisms.

The final answer is $\text{a) Humus}$.