Indian Geography

India is geologically and geographically enriched with a network of rivers like the Ganga, Yamuna, Brahmaputra, and others that flow perennially. These rivers carry mineral-rich sediments from the Himalayas and deposit them across the Indo-Gangetic plains, forming alluvial soil—some of the most fertile lands in the world. Therefore, the Assertion (A) is true and supported by physical geography and agricultural history.
The Amarakosa, an ancient Sanskrit thesaurus compiled by Amarasimha, is an important lexical and encyclopedic text that categorizes a wide range of topics including soils and types of land. It mentions twelve classifications of land based on characteristics such as fertility, irrigation, and physical appearance. Hence, Reason (R) is also factually accurate.
However, the Reason (R) does not directly explain the Assertion. The productivity of alluvial soil is due to the action of perennial rivers, not because of how the soil is described in classical texts. So while both statements are true, R is not the correct explanation of A.