The correct answer is Chennai. Chennai is the most populous district of Tamil Nadu according to the 2011 Census. It has a population of 4,681,086. Chennai is the capital of Tamil Nadu and the fourth most populous city in India. It is a major economic and cultural center in India.
Coimbatore is the second most populous district of Tamil Nadu with a population of 3,379,688. It is a major industrial city in India.
Madurai is the third most populous district of Tamil Nadu with a population of 3,191,116. It is a major cultural and religious center in India.
Tiruchhirappalli is the fourth most populous district of Tamil Nadu with a population of 2,901,871. It is a major industrial and educational center in India.