The correct answer is: Ajit Singh.
Ajit Singh was a Rajput ruler of the Loharu state in present-day Haryana, India. He was the son of Raja Jaswant Singh and grandson of Raja Suraj Mal. Ajit Singh built the Loharu Fort in 1733. The fort is located on a hilltop overlooking the town of Loharu. It is a large, imposing structure with thick walls and towers. The fort was built to protect the town from invaders. It was also used as a royal residence. The fort is now in ruins, but it is still an impressive sight.
Arjun Singh was a Rajput ruler of the Bikaner state in present-day Rajasthan, India. He was the son of Raja Karan Singh and grandson of Raja Rai Singh. Arjun Singh did not build the Loharu Fort.
Gurudutt Singh was a Rajput ruler of the Bharatpur state in present-day Rajasthan, India. He was the son of Maharaja Suraj Mal and grandson of Maharaja Jai Singh. Gurudutt Singh did not build the Loharu Fort.
Bhopal Singh was a Rajput ruler of the Jaipur state in present-day Rajasthan, India. He was the son of Maharaja Sawai Jai Singh II and grandson of Maharaja Sawai Jai Singh I. Bhopal Singh did not build the Loharu Fort.