Which best describes an iroquois longhouse a) a large, mound-shaped hollow in a steep hillside b) a circular tent covered with animal hides c) a 20x20-foot shelter built of wooden poles and bark d) a multistoried building of sun-dried bricks

Longhouses were homes that the Native Americans called Iroquois built and lived in. 
The answer is C).
The frames of the longhouses were made with poles which were covered with bark that was cut into rectangular slabs. A variety of different trees were used to build a longhouse, depending on the tree's strength, flexibility and resistance to decay. The roof of a typical Iroquois longhouse was rounded rather than peaked.
Sources: Study.com and www.warpaths2peacepipes.com