The correct answer is Patna. Patna is the capital of Bihar and the most populous city in the state. It is also a major railway junction, with several important railway lines passing through the city. The city is served by Patna Junction railway station, which is one of the busiest railway stations in India.
Gaya is another important city in Bihar, but it is not a major railway junction. The city is served by Gaya Junction railway station, which is a smaller railway station than Patna Junction.
Muzaffarpur is the third most populous city in Bihar, but it is not a major railway junction either. The city is served by Muzaffarpur Junction railway station, which is a smaller railway station than Patna Junction and Gaya Junction.
Darbhanga is the fourth most populous city in Bihar, and it is also not a major railway junction. The city is served by Darbhanga Junction railway station, which is a smaller railway station than Patna Junction, Gaya Junction, and Muzaffarpur Junction.