The correct answer is: A. Sirsa.
Sirsa is the biggest district of Haryana in terms of area. It has a total area of 4,421 square kilometers. It is located in the southern part of the state and is bordered by the states of Punjab and Rajasthan to the south and west, respectively. The district is also home to the Sutlej River, which flows through it from east to west.
Gurugram is the second biggest district of Haryana, with a total area of 4,040 square kilometers. It is located in the southern part of the state and is bordered by the states of Delhi and Rajasthan to the south and west, respectively. The district is also home to the National Capital Region (NCR), which is a major economic and industrial hub.
Rewari is the third biggest district of Haryana, with a total area of 2,880 square kilometers. It is located in the southern part of the state and is bordered by the states of Rajasthan and Uttar Pradesh to the south and west, respectively. The district is also home to the Aravalli Range, which runs through it from east to west.
Mahendragarh is the fourth biggest district of Haryana, with a total area of 2,730 square kilometers. It is located in the southern part of the state and is bordered by the states of Rajasthan and Uttar Pradesh to the south and west, respectively. The district is also home to the Narnaul Fort, which is a major tourist attraction.