The correct answer is: B. Bihar plain
The cold waves experienced in Bihar in winter season covers the Bihar plain. The Bihar plain is a vast, flat region that covers most of the state of Bihar. It is located in the Ganges Plain, which is a large, fertile plain that stretches across northern India and Bangladesh. The Bihar plain is a major agricultural region, and it is also home to a number of important cities, including Patna, the capital of Bihar.
The cold waves that occur in the Bihar plain are caused by the movement of cold air from the Himalayas. The cold air descends from the mountains and flows into the plain, causing the temperature to drop significantly. The cold waves can last for several days, and they can cause a number of problems, including power outages, crop damage, and health problems.
The other options are incorrect because they do not cover the Bihar plain. The Terai region is a narrow strip of land that lies between the Himalayas and the Ganges Plain. The Southern plateau uplands are a region of highland that lies in the southern part of India. The North-Eastern region is a region that lies in the northeastern part of India.