In Buddhist architecture, the prayer halls, usually with a stupa at one end, were called:

In Buddhist architecture, the "chaityas" were prayer halls or shrines, usually with a stupa at the far end.

About chaityas:

• A chaitya hall is usually long and rectangular, with a rounded (apsidal) end, where a small stupa (called a "dagoba") was placed.
• Rows of pillars divided the hall into a central nave and two side aisles, so that worshippers could walk around the stupa (circumambulation).
• The roof was usually vaulted (curved), imitating earlier wooden buildings, with wooden or stone ribs.
• The entrance had a large horseshoe-shaped window, called the "chaitya window" or "gavaksha", which let in light.
• Monks and lay people gathered there for prayer and worship.

Famous chaitya halls:

• Karla (Karle) caves, Maharashtra: the largest and finest rock-cut chaitya hall in India (1st century BCE - 1st century CE).
• Bhaja caves, Maharashtra: one of the earliest.
• Ajanta caves (Caves 9, 10, 19 and 26).
• Ellora (Cave 10, the "Vishvakarma" cave).
• Kanheri caves (Mumbai).
• Guntupalli (Andhra Pradesh): a circular chaitya.

Buddhist architecture:

• "Stupa": a relic mound.
• "Chaitya": a prayer hall.
• Monastery buildings with cells for monks also existed.
• "Stambha": pillars.

The word "chaitya":

It originally meant a sacred place or a shrine, including sacred trees and mounds.

Hindu architecture:

• "Mandapa": a pillared hall in front of the sanctum of a Hindu temple.
• "Gopuram": a part of south Indian temples.

Why not the other options?

• ✗Viharas are another type of Buddhist building.
• ✗Mandapas are pillared halls in Hindu temples.
• ✗Gopurams are gateway towers of south Indian temples.