Who had constructed the Jantar-Mantar at Delhi?

Concept: Maharaja Sawai Jai Singh II, the Kachhwaha Rajput ruler of Amber, was a Mughal-era scholar-administrator who corrected astronomical tables for Emperor Muhammad Shah. Between 1724 and 1735 he built five masonry observatories called Jantar Mantar (at Delhi, Jaipur, Ujjain, Varanasi and Mathura), using large fixed instruments such as the Samrat Yantra to track the positions of the sun, moon and planets with the naked eye.

Application: The Delhi Jantar Mantar, the first of the five, was completed around 1724 on open ground south of Shahjahanabad. Its construction followed a 1719 dispute at the Mughal court, when Hindu and Muslim astrologers could not agree on an auspicious date for an imperial expedition; Jai Singh II, already trusted for his astronomical expertise, was commissioned to build an observatory that could settle such calculations more reliably. This directly identifies Jay Singh-II as the builder.

Cross-check:

• Kirti Singh is documented as a Sisodia Rajput nobleman who held a jagir (feudal estate) in the Malwa region during the Mughal period, a minor local landholding unconnected to the Amber court that commissioned the Delhi observatory.

• Ajitmal is not a historical ruler at all; today it is a town and tehsil in Auraiya district, Uttar Pradesh, so it cannot be credited with commissioning any 18th-century monument.

• Vijay Singh ruled the separate Rajput kingdom of Marwar (Jodhpur) in two spells, 1752-1753 and 1772-1793, beginning nearly three decades after the Delhi Jantar Mantar was already built in 1724, and from a different royal house entirely.

Result: Only Jai Singh II's court, expertise and timeline (reign 1699-1743, overlapping the 1724 construction) match the historical record, confirming Jay Singh-II as the correct answer.