The correct answer is: C. Patna
Patna is the most populous district in Bihar, with a population of over 11 million people. It is the capital of Bihar and the second-largest city in India after Delhi. Patna is located on the banks of the Ganges River and is a major center for education, culture, and commerce.
Gaya is the second most populous district in Bihar, with a population of over 4 million people. It is a major pilgrimage site for Hindus and Buddhists. Gaya is located on the banks of the Phalgu River and is home to several important temples and monasteries.
Sheikhpura is the third most populous district in Bihar, with a population of over 2 million people. It is a rural district located in the south-central part of Bihar. Sheikhpura is known for its production of silk and rice.
Darbhanga is the fourth most populous district in Bihar, with a population of over 1.5 million people. It is a major agricultural district located in the north-central part of Bihar. Darbhanga is home to several important temples and mosques.