The correct answer is: A. Ganga
The Ganga is the longest river in India and the third-longest river in Asia. It flows through the states of Uttarakhand, Uttar Pradesh, Bihar, Jharkhand, West Bengal, and the union territory of Delhi. The Ganga has a drainage basin of about 1,000,000 square kilometers, which is about one-third of the area of India.
The Son is a river in India that flows through the states of Madhya Pradesh, Chhattisgarh, Jharkhand, and Bihar. It is a tributary of the Ganga. The Son has a drainage basin of about 100,000 square kilometers.
The Kiul is a river in India that flows through the states of Bihar and Jharkhand. It is a tributary of the Ganga. The Kiul has a drainage basin of about 10,000 square kilometers.
The Kosi is a river in India that flows through the states of Nepal and Bihar. It is a tributary of the Ganga. The Kosi has a drainage basin of about 90,000 square kilometers.
The Ganga is the largest river in Bihar by both length and drainage basin. It is also the most important river in Bihar, both economically and culturally. The Ganga is a major source of irrigation for the state, and it is also a major source of drinking water. The Ganga is also a major tourist attraction, and it is a sacred river for Hindus.