Final answer:
Document C is a primary source written by Pope Innocent IV during the time of the Mongols. The author of the document did not believe the world would be better off without the Mongols. Document C is considered the most reliable source because it provides firsthand information and insights into the author's perspective.
Step-by-step explanation:
Document C is a primary source because it is a letter that was written by Pope Innocent IV during the time of the Mongols. This can be determined because the document is written by someone who was alive during the events being discussed, and it provides firsthand information about the Mongols. Based on the document, the author of Document C did not believe that the world would be better off without the Mongols. The author mentions that the Mongols have brought many nations under their subjection and have gained power in many regions. Key pieces of evidence from Document C that support the claim that the author did not think the world would be better off without the Mongols include statements about the Mongols' rapid expansion and their ability to conquer nations. Document C seems to be the most reliable because it is a primary source that was written during the time of the Mongols, providing firsthand information and insights into the author's perspective.