Correct Option
The correct option is A rectangular building with a wagon-vaulted roof.
Explanation
Nagara temple architecture, prevalent in Northern India, is classified into three primary sub-types based on the shape of the shikhara (tower): Latina, Phamsana, and Valabhi. These styles are distinguished by their ground plans and the geometric form of their roofs.
Analysis of Options
- A rectangular building with a wagon-vaulted roof is Correct: The Valabhi sub-type is characterized by a rectangular base and a roof that rises into a vaulted chamber. This roof form is known as wagon-vaulted or barrel-vaulted, derived from the architectural style of bamboo or wooden carts and Buddhist Chaitya halls.
- A square base surmounted by a curvilinear tower is Incorrect: A square base surmounted by a curvilinear tower describes the Latina (or Rekha-Prasada) type, which is the most common form of Nagara temple.
- A pyramidal tower with stepped tiers is Incorrect: A pyramidal tower with stepped tiers describes the Phamsana type. Phamsana temples are broader and shorter than Latina ones, with roofs composed of slabs rising in a straight slope.
- A circular structure with a flat roof is Incorrect: Circular structures are rare in this context and are typically classified as Vritta, not Valabhi.
Key Takeaway: The Valabhi style of Nagara architecture is uniquely identified by its rectangular plan and wagon-vaulted (barrel-vaulted) roof.