The correct answer is: District Collector.
The District Collector is the administrative head of a district in Tamil Nadu. He/She is appointed by the Government of Tamil Nadu and is responsible for the overall administration of the district. The District Collector is also the District Magistrate and is responsible for law and order in the district.
The Divisional Commissioner is the head of a division, which is a group of districts. The Superintendent of Police is the head of the police force in a district. The Block Development Officer is the head of a block, which is a smaller administrative unit than a district.
The District Collector is the most senior of these officials and has the most authority. The Divisional Commissioner is responsible for coordinating the activities of the District Collectors in his/her division. The Superintendent of Police is responsible for maintaining law and order in the district. The Block Development Officer is responsible for implementing government schemes and programs at the block level.