Exp) Option a is the correct answer.

Early Buddhist art initially followed the aniconic tradition, where the Buddha was not depicted in human form. Instead, artists used various symbols to represent important events associated with the life and teachings of the Buddha.

In early Buddhist iconography, the empty seat symbolised the presence of the Buddha during his meditation and enlightenment. Rather than depicting the Buddha physically, sculptors represented him indirectly through symbolic motifs.

The empty throne or seat particularly signified the Buddha meditating under the Bodhi tree at Bodh Gaya, where he attained enlightenment.

Source: https://ncert.nic.in/textbook/pdf/lehs104.pdf  – Thinkers, Beliefs and Buildings Cultural Developments (c. 600 BCE – 600 CE) – Sculpture –  Symbols of worship – Page 100