The correct answer is Chennai.
Chennai is the most populous district in Tamil Nadu, with a population of over 4.6 million people. It is the capital of Tamil Nadu and the fourth most populous city in India. Chennai is a major economic and cultural center, and is home to many industries, including IT, manufacturing, and tourism.
Coimbatore is the second most populous district in Tamil Nadu, with a population of over 3.1 million people. It is a major industrial city, and is known for its production of textiles, automobiles, and engineering goods.
Madurai is the third most populous district in Tamil Nadu, with a population of over 2.6 million people. It is a major cultural center, and is home to many temples and historical monuments.
Tiruchhirappalli is the fourth most populous district in Tamil Nadu, with a population of over 1.9 million people. It is a major religious center, and is home to many temples and churches.