The correct answer is: a) Ganga River Basin.
The Ganga River Basin is the largest river basin in India, covering an area of about 1,000,000 square kilometers. It is home to over 400 million people, making it the most populous river basin in the world. The Ganga River is a sacred river for Hindus, and it is also an important source of water for irrigation, drinking, and hydroelectric power.
The Brahmaputra River Basin is the second largest river basin in India, covering an area of about 700,000 square kilometers. It is home to over 200 million people. The Brahmaputra River is a major source of water for irrigation, drinking, and hydroelectric power.
The Indus River Basin is the third largest river basin in India, covering an area of about 300,000 square kilometers. It is home to over 100 million people. The Indus River is a major source of water for irrigation, drinking, and hydroelectric power.
The Godavari River Basin is the fourth largest river basin in India, covering an area of about 1,000,000 square kilometers. It is home to over 300 million people. The Godavari River is a major source of water for irrigation, drinking, and hydroelectric power.
In conclusion, Bihar primarily falls within the Ganga River Basin.