The answer would be B: write down exactly who wrote the information and where it comes from
Documenting a source is another way to say Citing a source. When you cite a source you are telling where you got the info and who was the author of that info. When you don't cite a source that is called PLAGIARISING. Plagiarism is illegal and in most schools, there are huge consequences. So, when you use info that is not yours, make sure to cite it :)
Hope that helps!
-Chris