Correct Option (A)
Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
The frequency of floods in the North Indian plains has indeed increased over recent decades. This phenomenon is significantly linked to the reduction in the depth of river valleys, primarily caused by the extensive deposition of silt. Siltation reduces the carrying capacity of rivers, meaning they can hold less water within their banks. Consequently, during periods of heavy rainfall or increased water discharge, rivers are more prone to overflowing their banks, leading to more frequent and severe flooding in the surrounding plains.
Incorrect Options:
Options (B), (C), and (D) are incorrect because both the assertion and the reason are factually accurate, and the reason directly explains the assertion.
Option (B) is incorrect because the reduction in river valley depth due to silt deposition directly contributes to the increased frequency of floods by diminishing the river's capacity to contain water.
Option (C) is incorrect because both the increased flood frequency in North Indian plains and the reduction in river valley depth due to siltation are well-documented environmental observations.
Option (D) is incorrect as both the assertion and the reason are true statements regarding hydrological changes in the North Indian plains.