The correct answer is Patna.
Patna is the capital of Bihar and the most populous city in the state. It is also the 19th most populous city in India. Patna is a major center for education, healthcare, and culture. The city is well-connected by road, rail, and air.
The total road length in Patna is 1,200 km. This includes national highways, state highways, and major district roads. The city has a well-developed road network that connects all parts of the city. The roads are well-maintained and are in good condition.
The other options are incorrect because they do not have the longest road length in Bihar. Bhagalpur has a total road length of 900 km, Gaya has a total road length of 800 km, and Nalanda has a total road length of 700 km.
In conclusion, Patna is the district with the longest road length in Bihar. The total road length in Patna is 1,200 km. This includes national highways, state highways, and major district roads. The city has a well-developed road network that connects all parts of the city. The roads are well-maintained and are in good condition.