The depiction of the descent of the river Ganga from heaven can be found in the form of relief sculpture on rock at Mahabalipuram. Mahabalipuram, also known as Mamallapuram, is a historic town in Tamil Nadu, India. It is famous for its ancient rock-cut temples and monuments, many of which are UNESCO World Heritage Sites.

The relief sculpture at Mahabalipuram represents an important episode from Hindu mythology, known as the descent of the river Ganga. According to Hindu belief, the goddess Ganga descended from heaven to cleanse the sins of humanity. The sculpture depicts Ganga in the form of a river, flowing down from the heavens with her attendants.

It is important to note that the other options mentioned in the question do not feature the specific depiction of the descent of the river Ganga. Ellora is known for its impressive rock-cut caves and temples, but it does not have a sculpture specifically depicting the descent of Ganga. Sanchi is famous for its Buddhist monuments, while Amaravati is known for its ancient Buddhist stupa. These sites do not have the relief sculpture of Ganga`s descent. Therefore, the correct answer is option 2 - Mahabalipuram.