The earliest example of Buddhist stupa architecture has been found at Sarnath.

Explanation:
Sarnath is a historical and significant place for Buddhists as it is the site where Gautama Buddha first taught the Dharma after attaining enlightenment. It is located near Varanasi in Uttar Pradesh, India. The Buddhist stupa at Sarnath is considered to be one of the earliest and most important examples of this architectural form.

1. Significance of Sarnath in Buddhism:
- Sarnath is one of the four main pilgrimage sites for Buddhists, along with Lumbini (Buddha's birthplace), Bodh Gaya (site of Buddha's enlightenment), and Kushinagar (site of Buddha's death).
- After attaining enlightenment in Bodh Gaya, Buddha traveled to Sarnath and delivered his first sermon, known as the Dhammacakkappavattana Sutta or "Setting in Motion the Wheel of Dharma."

2. Stupa architecture:
- Stupas are monumental structures in Buddhism that contain relics or sacred objects associated with Buddha or his disciples. They symbolize various aspects of Buddhist philosophy and serve as important pilgrimage sites.
- Stupas typically have a hemispherical dome shape with a central core containing relics, surrounded by a circular platform and a raised circular railing called harmika.
- The harmika often contains a mast with three or more horizontal discs, representing the three jewels of Buddhism - Buddha, Dharma, and Sangha.
- Stupas are considered sacred and are objects of veneration and meditation for Buddhist followers.

3. Stupa at Sarnath:
- The stupa at Sarnath is known as Dhamek Stupa and is believed to have been built by Emperor Ashoka in the 3rd century BCE.
- It is made of stone and brick, with a cylindrical base and a height of around 43.6 meters.
- The stupa is adorned with intricate carvings depicting scenes from Buddha's life and various Buddhist symbols.
- The Dhamek Stupa is an important pilgrimage site for Buddhists and attracts devotees from around the world.

In conclusion, the earliest example of Buddhist stupa architecture has been found at Sarnath, where the Dhamek Stupa stands as a testament to the rich heritage and symbolism of Buddhism.