The correct answer is option 'C' - Bay of Bengal.

Nor'westers, also known as Northwesterly thunderstorms, are severe thunderstorms that occur in the Indian subcontinent, particularly in the eastern parts of Bihar, West Bengal, and Bangladesh. These thunderstorms bring strong winds, heavy rains, and sometimes hailstones.

Here is a detailed explanation of why Nor'westers mostly affect East Bihar and their origin from the Bay of Bengal:

The Bay of Bengal:
The Bay of Bengal is a large body of water located in the northeastern part of the Indian Ocean. It is bounded by India, Bangladesh, Myanmar, and Sri Lanka. The bay is known for its warm waters and is susceptible to the formation of cyclones and other weather disturbances.

Formation of Nor'westers:
Nor'westers are typically formed during the pre-monsoon season, which occurs from March to May. During this period, the landmass of India and Bangladesh starts heating up due to the increasing intensity of the sun's rays. As a result, hot air rises rapidly from the surface, creating a low-pressure area.

The Bay of Bengal plays a crucial role in the formation of Nor'westers. The warm and moist air from the bay converges with the hot air over the land, leading to the development of thunderstorms. The convergence of these air masses creates a favorable environment for the formation of severe thunderstorms.

Movement towards East Bihar:
Once formed, these Nor'westers move in a northeasterly direction, driven by the prevailing winds. The winds associated with Nor'westers blow from the northwest, hence the name. As these thunderstorms move across the Bay of Bengal towards the coast, they gather moisture and intensify.

East Bihar is located in the path of these thunderstorms. The state is situated in the eastern part of India, adjacent to West Bengal and Bangladesh, which are also frequently affected by Nor'westers. The proximity of East Bihar to the Bay of Bengal makes it vulnerable to these severe thunderstorms.

Impact on East Bihar:
When Nor'westers reach East Bihar, they bring strong winds, heavy rainfall, and sometimes hailstones. These thunderstorms can cause significant damage to buildings, infrastructure, agriculture, and can also result in loss of life. The heavy rainfall associated with Nor'westers can lead to flooding, further exacerbating the impact on the region.

In conclusion, Nor'westers mostly affect East Bihar, and their origin can be traced back to the Bay of Bengal. The convergence of warm and moist air from the bay with hot air over the land leads to the formation of severe thunderstorms. These thunderstorms move in a northeasterly direction, impacting East Bihar and other regions in their path. The Bay of Bengal's warm waters and favorable atmospheric conditions contribute to the formation and intensity of Nor'westers.