Concept:Jantar Mantar in Delhi is one of five astronomical observatories built by Maharaja Jai Singh II of Jaipur in 1724.
Explanation:Mughal emperor Muhammad Shah assigned Jai Singh II to revise the calendar and astronomical records.
Jai Singh II then constructed five observatories at Delhi, Jaipur, Mathura, Ujjain, and Varanasi.
The Delhi Jantar Mantar was built in 1724.
Among these, the Jaipur observatory is the largest.
Other rulers built different monuments: Akbar built Buland Darwaza, Shah Jahan built Taj Mahal and Red Fort, and Sawai Pratap Singh built Hawa Mahal.
Thus, the correct builder of Delhi’s Jantar Mantar is Jay Singh-II (Jai Singh II).
Answer:Option D: Jay Singh-II