C. These early doughnuts were found in prehistoric Native American ruins.
C is the best answer. A tradition is something that has been around for a while. Option C says that doughnuts were found in prehistoric Native American ruins which means that they have been around for a long long time. Option D is not a good choice because it doesn't say anything about donuts, it's only about bread. The other options do not indicate that they have been around for a significant portion of time.