The winter rains caused by Western disturbance in North Westem Plain of India gradually decreases from

1. A

   East to West

   East-to-west would mean more winter rain toward Rajasthan than Punjab, which is the opposite of the western-disturbance gradient.

2. B

   West to East

   Western disturbances enter from the west/north-west. Winter rain and snowfall in the north-western plains therefore decrease from west to east (Punjab–Haryana–western UP get more than eastern UP/Bihar). That is the official key.

3. C

   North to South

   A simple north-to-south decrease is not the standard statement for the plains gradient of western-disturbance rain.

4. D

   South to North

   South-to-north is the reverse of Himalayan orographic increase and is not the keyed plains pattern.

Summary. Official key is (b). Western-disturbance winter rain on the north-western plains decreases west to east.