The correct answer is: a) District Magistrate.
The District Magistrate is the chief executive officer of the district. He/She is responsible for the maintenance of law and order, revenue collection, and implementation of government schemes. The District Magistrate is also the chairman of the District Disaster Management Authority.
The District Collector is the head of the revenue department in the district. He/She is responsible for the collection of land revenue, registration of documents, and maintenance of land records. The District Collector is also the chairman of the District Land Tribunal.
The Superintendent of Police is the head of the police department in the district. He/She is responsible for the maintenance of law and order, investigation of crimes, and prevention of crime. The Superintendent of Police is also the chairman of the District Crime Prevention Committee.
The Zilla Parishad Chairperson is the head of the Zilla Parishad, which is the local self-government body at the district level. The Zilla Parishad is responsible for the development of the district in areas such as education, health, agriculture, and rural roads.
The District Magistrate is the most senior of the four officials mentioned in the question. He/She is also the only one who is responsible for the maintenance of law and order. Therefore, the District Magistrate is the correct answer.