The correct answer is option C (citations).
MLA, APA, and CMS are the standard conventions used to CITE the work of others in order to avoid being accused of plagiarism.
The conventions authors use depend on the type of work they carry out.
For example, MLA (Modern Language Association) is very used in literature and humanities in general, APA (American Psychological Association) is more common for social sciences, and CMS (Chicago Manual of Style) is accurate to give format to written works that will be published.
All of them refer to CITATIONS.