Jantar Mantar was built by -

Correct answer: (d) Maharaja Jai Singh

Explanation

The Jantar-Mantar in Jaipur was built by Sawai Raja Jai Singh. It is located near the City Palace. The main instruments at the Jantar-Mantar observatory include Samrat Yantra, Ram Yantra, Rashi-Valaya Yantra, and Kapali Yantra, which accurately indicate the motion of the sun and the moon and the positions of stars. Notably, for research in astronomy and celestial science, Sawai Jai Singh built five observatories at Delhi, Mathura, Ujjain, Varanasi, and Jaipur, among which the Jaipur observatory is the largest and highly scientific in design.