The correct answer is A.
Plagarism consists on introducing in your work parts of the work previously elaborated by another person, without acknowledging his/her merit through citations. Every piece of information which is not produced by the individual who is writing, for example, a scientific paper needs to be properly cited and referenced, giving the recognition to the real author, in order to prevent plagiarism.
Including cites and references in a written text are processes which consist on mentioning the author of each idea described, together with the title of paper, book or article where he mentioned it, the year of publication and some other details.
