The correct answer is (d). Maharaja Sawai Jai Singh of Jaipur had not built the observatory at Allahabad.
Maharaja Sawai Jai Singh II (1688-1743) was the ruler of the Indian state of Jaipur from 1707 to 1743. He was a great patron of astronomy and mathematics, and he built five observatories in India, including the Jantar Mantar in Jaipur. The other four observatories are located at Ujjain, Varanasi, Mathura, and Delhi.
The Jantar Mantar in Jaipur is the largest and best-preserved of the five observatories. It is a collection of astronomical instruments, including sundials, astrolabes, and armillary spheres. The instruments are used to measure the position of the sun, moon, and stars.
The Jantar Mantar in Jaipur is a UNESCO World Heritage Site. It is a popular tourist destination, and it is also used by astronomers for research.