A writer can show that a section of writing came from a different source primarily by using C: citation. Citing sources indicates to the reader where the information originated and gives credit to the original authors, which is essential for academic integrity and avoiding plagiarism.
While A (gathering information from a variety of sources) and D (paraphrasing) can also play a role in incorporating different perspectives and ideas into writing, they don't specifically identify where a particular section came from. B (avoiding plagiarism) is an important principle but is more about the practice of proper attribution rather than a method of showing the source itself.
In summary, using citations is the most direct and effective way to indicate that information comes from a different source.