A thesaurus is a reference work to find synonyms of words, and that doesn't have anything to do with what we're trying to find, so it's incorrect
An encyclopedia is a book that gives info on many subjects, and that has nothing to do with what we're trying to find, so it's incorrect
A dictionary is recourse to find definitions of words, and that doesn't have to do with what we're trying to find, so it's incorrect
A journal entry is exactly what is sounds like. Someone owned a journal and wrote in it. And, the journal comes directly from the time it was used. It is a primary source. Therefore, that is the correct answer.
