Sources must be cited in order to
a. paraphrase.
b. provide supporting evidence.
c. summarize information.
d. avoid plagiarism.

Sources must be cited in order to avoid plagiarism. The answer's D. avoid plagiarism. It is important to cite the sources to give credits to whom/where it is due or else you will be charge of theft. The terms “borrowing” and “copying” the idea you get from books, in the internet, or other source materials might be a disguise to some serious offense. It is a crime to copy or steal someone’s idea or work of others because it is considered as an intellectual property and it is protected by the copyright law. This is what plagiarism is, stealing one’s idea and not citing the sources right.