1. Assertion (A): The frequency of floods in North Indian plains has increased during the last couple of decades. 2. Reason (R): There has been reduction in the depth of river valleys due to deposition of silt.

1. Assertion (A) is true. Reports and data leading up to the year 2000 indicated an increasing trend in the frequency and intensity of floods in the North Indian plains, particularly in river basins like the Ganga and Brahmaputra. 2. Reason (R) is true. Increased deposition of silt in river valleys, often exacerbated by deforestation in catchment areas and erosion, leads to the raising of riverbeds and a reduction in the depth and carrying capacity of river channels. This makes rivers more prone to overflowing their banks during periods of high discharge. 3. The reduction in river channel capacity due to siltation (R) is a direct physical cause contributing to the increased frequency of floods (A). Therefore, R is the correct explanation of A.