The correct answer is D. It's borrowing without giving the original author credit.
Plagiarism is a serious academic offense, which is why C is incorrect - if you plagiarize, meaning steal another person's work, you can be sued so it's better to avoid doing that. A is also incorrect because it can appear in any writing, not just scholarly. B is incorrect because it's easier to just give credit to the original author because you might not be good at paraphrasing.