The capital city of Bihar is:

Gaya
Patna
Muzaffarpur
Bhagalpur

The correct answer is: b) Patna.

Patna is the capital of the Indian state of Bihar. It is the 19th most populous city in India and the 13th most populous urban agglomeration in India. Patna is the seat of the Patna High Court and the Patna University. It is also home to the Gandhi Maidan, one of the largest parks in India.

Gaya is a city in the Indian state of Bihar. It is the headquarters of the Gaya district and the Gaya division. Gaya is a major pilgrimage site for Hindus and Buddhists. The city is home to the Mahabodhi Temple, a UNESCO World Heritage Site.

Muzaffarpur is a city in the Indian state of Bihar. It is the headquarters of the Muzaffarpur district and the Muzaffarpur division. Muzaffarpur is a major commercial and educational center in Bihar. The city is home to the University of Agriculture and Technology, Muzaffarpur.

Bhagalpur is a city in the Indian state of Bihar. It is the headquarters of the Bhagalpur district and the Bhagalpur division. Bhagalpur is a major industrial and educational center in Bihar. The city is home to the Tilka Manjhi Bhagalpur University.