Da Parbatia, near Tezpur, showcases a unique example of:

The correct answer is: a) Buddhist monastery ruins.

Da Parbatia is a 7th-century Buddhist monastery located near Tezpur, Assam, India. It is one of the oldest and most important Buddhist sites in the region. The monastery is built on a hilltop and consists of a series of stone terraces, stupas, and temples. The ruins of the monastery are well-preserved and provide a valuable insight into the history of Buddhism in Assam.

The other options are incorrect because:

• Option b) is incorrect because Gupta-era temple architecture is not found in Da Parbatia.
• Option c) is incorrect because a doorframe from an early temple is not found in Da Parbatia.
• Option d) is incorrect because a medieval fort is not found in Da Parbatia.