Final answer:
The Rangers likely found an ancient artifact beneath the yellow tarp, based on archaeological practices and finds similar to ancient hunting methods and tools uncovered in other dig sites.
Step-by-step explanation:
The question about what the Rangers found beneath the yellow tarp is a bit ambiguous without further context. However, based on the information provided and the nature of archaeological discoveries, it seems that what is often found during such excavations are ancient artifacts or remnants indicating past human activities, such as tools, weapons, or even clothing from a bygone era.
The reference to an ancient method of hunting bison found in Texas and Montana, as well as the mention of a 9,000-year-old grave with a hunting toolkit, points towards the possibility that the Rangers might have found something similar, such as an ancient artifact. Despite the various possibilities hinted at within the context - fossilized firewood, tree ring data, medieval clothing - none of them directly relate to the options provided in the question.