The north-eastern states of India receive the highest annual rainfall from the southwest monsoon. This is due to their hilly terrain and proximity to the Bay of Bengal, which results in heavy rainfall in this region.

The correct answer is option 'C', the north-eastern states of India receive the highest annual rainfall from the southwest monsoon. This region is known for its high precipitation levels and is often referred to as the "rainiest region" of India.

Reasons for the highest annual rainfall in the north-eastern states:

1. Geographic Location:
- The north-eastern states are located in the eastern part of India, bordered by the Himalayas in the north and Bangladesh in the south.
- This region is surrounded by hills and mountains, which act as barriers to the monsoon winds, causing them to rise and cool, leading to condensation and ultimately heavy rainfall.

2. Orographic Effect:
- The orographic effect plays a significant role in the high rainfall received by the north-eastern states.
- The monsoon winds, laden with moisture, are forced to rise as they encounter the Eastern Himalayas.
- As the air rises, it cools and condenses, resulting in the formation of clouds and subsequent rainfall.

3. Bay of Bengal Branch of Southwest Monsoon:
- The north-eastern states are primarily influenced by the Bay of Bengal branch of the southwest monsoon.
- Moisture-laden winds from the Bay of Bengal traverse over the region, leading to abundant rainfall.
- The Bay of Bengal is known for its warm sea surface temperatures, which provide the necessary moisture for the formation of rain clouds.

4. Relief Features:
- The relief features of the north-eastern states, characterized by hills, valleys, and plateaus, contribute to the rainfall pattern.
- The hills act as barriers, forcing the moist winds to rise and cool, which leads to enhanced precipitation.
- Additionally, the valleys and plateaus provide suitable conditions for the formation of clouds and rainfall.

5. Confluence of Wind Systems:
- The north-eastern states experience the convergence of multiple wind systems, namely the Bay of Bengal branch of the southwest monsoon, the south-westerly winds from the Arabian Sea, and the north-easterly winds from the Indian subcontinent.
- This convergence results in the upliftment of air masses, leading to the condensation of moisture and subsequent rainfall.

Therefore, due to its geographical location, orographic effect, influence of the Bay of Bengal branch of the southwest monsoon, relief features, and the confluence of wind systems, the north-eastern states of India receive the highest annual rainfall from the southwest monsoon.