The correct answer is D. Jantar Mantar.
The Red Fort, Humayun’s Tomb, and Qutub Minar are all included as World Heritage Sites by UNESCO. Jantar Mantar is an astronomical observatory complex built by Maharaja Jai Singh II of Jaipur in the 18th century. It is one of five such observatories built by Jai Singh, and is the best-preserved of them. The observatory consists of a number of instruments used to measure time, the position of the sun, moon, and stars, and the planets. It is a unique and important example of 18th-century Indian architecture and astronomy.
The Red Fort is a 17th-century Mughal palace fort in the city of Delhi, India. It was commissioned by the Mughal emperor Shah Jahan in 1638, and was completed in 1648. The fort is made of red sandstone, and is one of the largest and most impressive buildings in Delhi. It is a UNESCO World Heritage Site.
Humayun’s Tomb is a 16th-century Mughal mausoleum located in Delhi, India. It was commissioned by Humayun’s widow, Bega Begum, in 1562, and was completed in 1572. The tomb is one of the earliest examples of Mughal architecture, and is considered to be one of the finest examples of the style. It is a UNESCO World Heritage Site.
Qutub Minar is a minaret that forms part of the Qutb complex, a UNESCO World Heritage Site in the Mehrauli area of Delhi, India. It is a 73-metre (240 ft) tall tapering tower of red sandstone and marble, and is the tallest brick minaret in the world. The Qutub Minar was commissioned by Qutb-ud-din Aibak, the founder of the Delhi Sultanate, in 1193, and was completed by his successor, Iltutmish, in 1230.