Which is the oldest Stupa of the Buddhist Period?

Correct option is D

The correct answer is (d) Sanchi Stupa.
 The Sanchi Stupa in Madhya Pradesh, India is one of the oldest and best-preserved Buddhist stupas, built in the 3rd century BCE by Emperor Ashoka.
 The stupa is situated atop a hill in the small village of Sanchi and is surrounded by a complex of other Buddhist structures.
 It's over 2,200 years old and is also the oldest stone structure in India
Facts to remember:
 Kesariya Stupa, also known as Kesariya Chaitya, is located in the Kesariya village of Bihar, India, and dates back to the 3rd century BCE, during the time of Emperor Ashoka.
 The Mahabodhi Stupa, also known as the Great Stupa, is an ancient Buddhist monument located in Bodh Gaya, Bihar, India.
 It is one of the most sacred sites in Buddhism as it marks the spot where Siddhartha Gautama, the historical Buddha, attained enlightenment under the Bodhi tree.