Final answer:
The buffalo hide was used to create bullboats by stretching it over a framework of willow branches, creating a durable and waterproof vessel used by Indigenous peoples.
Step-by-step explanation:
The buffalo part that was stretched out to create bullboats is B. Hide. Bullboats were a form of watercraft used by various Indigenous peoples, particularly those living in the plains regions of North America. To construct a bullboat, buffalo hide was stretched over a frame made of willow branches or other flexible wood. This craft was essential for crossing and traveling on rivers, and the buffalo's hide provided a durable and waterproof material.