The correct answer is B) libraries.
The best place to find primary sources is in the library.
A primary source is a document that gives you first-hand evidence or testimony of thing that happened in the past. It could be testimonies of people that lived in those moments or events or pictures or video taken by people from that time, or original documents sealed or signed by the people who elaborated them.
All of these resources can be found in a library. Books, magazines, newspaper, videos, probably audios, and computers to search on the internet.