The correct answer is: Both are correct and R is the correct explanation of A.
Western disturbances are extratropical storms that form over the Mediterranean Sea and move eastward across the Middle East and into the Indian subcontinent. They are caused by a difference in pressure between the cold air masses over the Mediterranean Sea and the warm air masses over the Indian subcontinent. This difference in pressure creates a low-pressure system, which draws in moist air from the Mediterranean Sea. As this air rises, it cools and condenses, forming clouds and precipitation.
Bihar plain experiences Western disturbances originating from Mediterranean Sea between December to February. This is because the pressure difference between the cold air masses over the Mediterranean Sea and the warm air masses over the Indian subcontinent is greatest during this time of year. The Western disturbances that form during this time of year are also the most intense, which is why they bring the most precipitation to Bihar plain.
The reason (R) is the correct explanation of the assertion (A). The Western disturbances originate due to pressure differences, and these pressure differences are greatest between December to February. This is why Bihar plain experiences Western disturbances originating from Mediterranean Sea between December to February.