Which of the following is the oldest Buddhist monument structure in India?

The Great Stupa (heap) at Sanchi is the oldest stone structure of its kind and was commissioned by the Maurya emperor Ashoka the Great in the 3rd century BCE. It contains the relics of the Buddha and is decorated with fine examples of Pre-Iconic Buddhist scripture. With a height of 16.46 m (54 ft) and a pedestal diameter of 36.6 m (120 ft), it is one of the largest of its kind in the whole country. It was adorned with three "Chhatris" (umbrella-like structures in Hindi language) meant to shelter the relics. The three structures are said to stand for Triantha or the jewels of Buddhism, namely the Buddha, the Dharma, and the Sangha. A large central pillar supports the "Chhatris". Currently, there are around fifty additional structures on the hilltop.

Sanchi is a Buddhist complex, famous for its Great Stupa, on a hill top at Sanchi Town in Raisen District of the State of Madhya Pradesh, India. It is located in 46 kilometers (29 miles) north-east of Bhopal, capital of Madhya Pradesh. The Great Stupa is a UNESCO (The United Nations Educational, Scientific and Cultural Organization) "World Heritage Site". It is believed that Ashoka's intention behind constructing this Stupa was to preserve and spread the Buddhist philosophy and way of life.

In 1989, the Great Stupa and other Buddhist structures at Sanchi were collectively declared a World Heritage site by UNESCO for their cultural significance. Buddhist monks use to come here for meditation and pilgrimage.