Concept:
Bihar is closely associated with Buddhism, Jainism, and ancient Indian history.
Many Buddhist monuments, stupas, monasteries, and sacred sites are found in Bihar.
Step 1: Understand the historical importance of Bihar.
Bihar was an important centre of ancient Indian civilization.
Places like Bodh Gaya, Nalanda, Rajgir, and Vaishali are located in Bihar.
These places are connected with Buddhism and Jainism.
Step 2: Connect Bihar with Buddhist monuments.
Stupas are important Buddhist monuments.
They are built to preserve relics or commemorate important events related to Buddhism.
Bihar is famous for such Buddhist heritage and stupa tradition.
Step 3: Final conclusion.
Thus, Bihar is famous for:
\[
\boxed{\text{Stupa}}
\]