The correct answer is: A. Bhagalpur
The Ganga river flows through the districts of Bhagalpur, Munger, Patna, and Buxar in Bihar. The length of the Ganga river in Bhagalpur district is 120 kilometers. The length of the Ganga river in Munger district is 100 kilometers. The length of the Ganga river in Patna district is 70 kilometers. The length of the Ganga river in Buxar district is 50 kilometers.
Therefore, the district of Bihar which has a longer length of Ganga river is Bhagalpur.
Option B: Katihar is a district in the Indian state of Bihar. The Ganga river does not flow through Katihar district.
Option C: Patna is the capital of the Indian state of Bihar. The Ganga river flows through Patna district. However, the length of the Ganga river in Patna district is shorter than the length of the Ganga river in Bhagalpur district.
Option D: Begusarai is a district in the Indian state of Bihar. The Ganga river flows through Begusarai district. However, the length of the Ganga river in Begusarai district is shorter than the length of the Ganga river in Bhagalpur district.
Option E: None of the above is correct.