The correct answer is: a) Secret underground structure.
The Talatal Ghar is a secret underground structure located in Sibsagar, Assam, India. It was built in the 17th century by the Ahom kings. The structure is made of bricks and is about 100 feet deep. It has a number of rooms and passages, and its purpose is still unknown. Some believe that it was used as a treasury, while others believe that it was used as a place of worship.
Option b) is incorrect because the Talatal Ghar is not a multi-story palace. It is a single-story structure.
Option c) is incorrect because the Talatal Ghar is not an astronomical observatory. There is no evidence that it was ever used for astronomical purposes.
Option d) is incorrect because the Talatal Ghar is not a temple complex. It is a single structure, not a complex of structures.