Why do historians have to include a citation when paraphrasing an idea from another source ?

The most common reason is to avoid plagiarism. Plagiarism is taking someone else’s ideas and presenting them as your own. Plagiarism needs to be avoided for two main reasons:

Another reason for citing is that it helps us look at the progression of information. This can be useful if we are interesting in seeing how ideas spread or how ways of thinking change through time.

i think your answer there is c

would not cooperate with each other voluntarily.