Which of the following is most likely to be considered plagiarism?

Using materials from a source without proper citation is considered plagiarism because it involves presenting someone else's ideas, words, or research findings as your own. Proper citation gives credit to the original author and acknowledges their contribution to the body of knowledge. When you utilize someone else's work without appropriate acknowledgement, you fail to recognize their intellectual property rights, which is the essence of plagiarism.

In contrast, summarizing a work in your own words can be acceptable as long as you accurately represent the original ideas and provide proper citation. Quoting a source with appropriate attribution is a legitimate way to incorporate someone else's words while giving them credit. Collaborating on a research paper is typically a form of teamwork and, if all contributions are appropriately acknowledged, it does not constitute plagiarism. Therefore, the lack of citation when using materials from a source is what primarily defines plagiarism.