Assertion: It was difficult for Art historians to interpret the various elements shown in Buddhist art.

Reason: The empty seat was meant to indicate the meditation of the Buddha.

Both the Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Art historians had to acquire familiarity with the hagiographies of the Buddha in order to understand Buddhist sculpture. According to hagiographies, the Buddha attained enlightenment while meditating under a tree. Many early sculptors did not show the Buddha in human form – instead, they showed his presence through symbols. The empty seat was meant to indicate the meditation of the Buddha. Another frequently used symbol was the wheel. This stood for the first sermon of the Buddha, delivered at Sarnath.
As is obvious, such sculptures cannot be understood literally – for instance, the tree does not stand simply for a tree, but symbolises an event in the life of the Buddha. In order to
understand such symbols, historians have to familiarise themselves with the traditions of those who produced these works of art.